diff --git a/HISTORY.md b/HISTORY.md
index 78d89beb1e..3214f9e566 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -7,6 +7,29 @@
   (at the time the release is made). If you need a dependency version increased,
   please open an issue and we can update it and make a new Catalyst release once
   testing against the newer dependency version is complete. 
+- New formula for inferring variables from equations (declared using the `@equations` options) in the DSL. The order of inference of species/variables/parameters is now:
+    (1) Every symbol explicitly declared using `@species`, `@variables`, and `@parameters` are assigned to the correct category.
+    (2) Every symbol used as a reaction reactant is inferred as a species.
+    (3) Every symbol not declared in (1) or (2) that occurs in an expression provided after `@equations` is inferred as a variable.
+    (4) Every symbol not declared in (1), (2), or (3) that occurs either as a reaction rate or stoichiometric coefficient is inferred to be a parameter.
+E.g. in
+```julia
+@reaction_network begin
+    @equations V1 + S ~ V2^2
+    (p + S + V1), S --> 0
+end
+```
+`S` is inferred as a species, `V1` and `V2` as variables, and `p` as a parameter. The previous special cases for the `@observables`, `@compounds`, and `@differentials` options still hold. Finally, the `@require_declaration` options (described in more detail below) can now be used to require everything to be explicitly declared.
+- New formula for determining whether the default differentials have been used within an `@equations` option. Now, if any expression `D(...)` is encountered (where `...` can be anything), this is inferred as usage of the default differential D. E.g. in the following equations `D` is inferred as a differential with respect to the default independent variable:
+```julia
+@reaction_network begin
+  @equations D(V) + V ~ 1
+end
+@reaction_network begin
+  @equations D(D(V)) ~ 1
+end
+```
+Please note that this cannot be used at the same time as `D` is used to represent a species, variable, or parameter (including is these are implicitly designated as such by e.g. appearing as a reaction reactant). 
 - Array symbolics support is more consistent with ModelingToolkit v9. Parameter
   arrays are no longer scalarized by Catalyst, while species and variables
   arrays still are (as in ModelingToolkit). As such, parameter arrays should now
diff --git a/docs/src/inverse_problems/examples/ode_fitting_oscillation.md b/docs/src/inverse_problems/examples/ode_fitting_oscillation.md
index ff3fb63805..a72ebd6b44 100644
--- a/docs/src/inverse_problems/examples/ode_fitting_oscillation.md
+++ b/docs/src/inverse_problems/examples/ode_fitting_oscillation.md
@@ -56,7 +56,7 @@ function optimise_p(pinit, tend)
         newprob = remake(prob; tspan = (0.0, tend), p = p)
         sol = Array(solve(newprob, Rosenbrock23(); saveat = newtimes))
         loss = sum(abs2, sol .- sample_vals[:, 1:size(sol,2)])
-        return loss, sol
+        return loss
     end
 
     # optimize for the parameters that minimize the loss
diff --git a/src/dsl.jl b/src/dsl.jl
index a2bf01ec41..ac1bb88de9 100644
--- a/src/dsl.jl
+++ b/src/dsl.jl
@@ -290,7 +290,7 @@ struct UndeclaredSymbolicError <: Exception
     msg::String
 end
 
-function Base.showerror(io::IO, err::UndeclaredSymbolicError) 
+function Base.showerror(io::IO, err::UndeclaredSymbolicError)
     print(io, "UndeclaredSymbolicError: ")
     print(io, err.msg)
 end
@@ -328,11 +328,6 @@ function make_reaction_system(ex::Expr; name = :(gensym(:ReactionSystem)))
     parameters_declared = extract_syms(options, :parameters)
     variables_declared = extract_syms(options, :variables)
 
-    # Reads equations.
-    vars_extracted, add_default_diff, equations = read_equations_options(
-        options, variables_declared; requiredec)
-    variables = vcat(variables_declared, vars_extracted)
-
     # Handle independent variables
     if haskey(options, :ivs)
         ivs = Tuple(extract_syms(options, :ivs))
@@ -352,16 +347,21 @@ function make_reaction_system(ex::Expr; name = :(gensym(:ReactionSystem)))
         combinatoric_ratelaws = true
     end
 
-    # Reads observables.
-    observed_vars, observed_eqs, obs_syms = read_observed_options(
-        options, [species_declared; variables], all_ivs; requiredec)
-
     # Collect species and parameters, including ones inferred from the reactions.
     declared_syms = Set(Iterators.flatten((parameters_declared, species_declared,
-        variables)))
+        variables_declared)))
     species_extracted, parameters_extracted = extract_species_and_parameters!(
         reactions, declared_syms; requiredec)
 
+    # Reads equations (and infers potential variables).
+    # Excludes any parameters already extracted (if they also was a variable).
+    declared_syms = union(declared_syms, species_extracted)
+    vars_extracted, add_default_diff, equations = read_equations_options(
+        options, declared_syms, parameters_extracted; requiredec)
+    variables = vcat(variables_declared, vars_extracted)
+    parameters_extracted = setdiff(parameters_extracted, vars_extracted)
+
+    # Creates the finalised parameter and species lists.
     species = vcat(species_declared, species_extracted)
     parameters = vcat(parameters_declared, parameters_extracted)
 
@@ -369,6 +369,10 @@ function make_reaction_system(ex::Expr; name = :(gensym(:ReactionSystem)))
     diffexpr = create_differential_expr(
         options, add_default_diff, [species; parameters; variables], tiv)
 
+    # Reads observables.
+    observed_vars, observed_eqs, obs_syms = read_observed_options(
+        options, [species_declared; variables], all_ivs; requiredec)
+
     # Checks for input errors.
     (sum(length.([reaction_lines, option_lines])) != length(ex.args)) &&
         error("@reaction_network input contain $(length(ex.args) - sum(length.([reaction_lines,option_lines]))) malformed lines.")
@@ -701,7 +705,7 @@ end
 # `vars_extracted`: A vector with extracted variables (lhs in pure differential equations only).
 # `dtexpr`: If a differential equation is defined, the default derivative (D ~ Differential(t)) must be defined.
 # `equations`: a vector with the equations provided.
-function read_equations_options(options, variables_declared; requiredec = false)
+function read_equations_options(options, syms_declared, parameters_extracted; requiredec = false)
     # Prepares the equations. First, extracts equations from provided option (converting to block form if required).
     # Next, uses MTK's `parse_equations!` function to split input into a vector with the equations.
     eqs_input = haskey(options, :equations) ? options[:equations].args[3] : :(begin end)
@@ -713,34 +717,40 @@ function read_equations_options(options, variables_declared; requiredec = false)
     # Loops through all equations, checks for lhs of the form `D(X) ~ ...`.
     # When this is the case, the variable X and differential D are extracted (for automatic declaration).
     # Also performs simple error checks.
-    vars_extracted = Vector{Symbol}()
+    vars_extracted = OrderedSet{Union{Symbol, Expr}}()
     add_default_diff = false
     for eq in equations
         if (eq.head != :call) || (eq.args[1] != :~)
             error("Malformed equation: \"$eq\". Equation's left hand and right hand sides should be separated by a \"~\".")
         end
 
-        # Checks if the equation have the format D(X) ~ ... (where X is a symbol). This means that the
-        # default differential has been used. X is added as a declared variable to the system, and
-        # we make a note that a differential D = Differential(iv) should be made as well.
-        lhs = eq.args[2]
-        # if lhs: is an expression. Is a function call. The function's name is D. Calls a single symbol.
-        if (lhs isa Expr) && (lhs.head == :call) && (lhs.args[1] == :D) &&
-           (lhs.args[2] isa Symbol)
-            diff_var = lhs.args[2]
-            if in(diff_var, forbidden_symbols_error)
-                error("A forbidden symbol ($(diff_var)) was used as an variable in this differential equation: $eq")
-            elseif (!in(diff_var, variables_declared)) && requiredec
-                throw(UndeclaredSymbolicError(
-                                              "Unrecognized symbol $(diff_var) was used as a variable in an equation: \"$eq\". Since the @require_declaration flag is set, all variables in equations must be explicitly declared via @variables, @species, or @parameters."))
-            else
-                add_default_diff = true
-                in(diff_var, variables_declared) || push!(vars_extracted, diff_var)
-            end
+        # If the default differential (`D`) is used, record that it should be decalred later on.
+        if (:D ∉ union(syms_declared, parameters_extracted)) && find_D_call(eq)
+            requiredec && throw(UndeclaredSymbolicError(
+                "Unrecognized symbol D was used as a differential in an equation: \"$eq\". Since the @require_declaration flag is set, all differentials in equations must be explicitly declared using the @differentials option."))
+            add_default_diff = true
+            push!(syms_declared, :D)
         end
+
+        # Any undecalred symbolic variables encountered should be extracted as variables.
+        add_syms_from_expr!(vars_extracted, eq, syms_declared)
+        (!isempty(vars_extracted) && requiredec) && throw(UndeclaredSymbolicError(
+            "Unrecognized symbolic variables $(join(vars_extracted, ", ")) detected in equation expression: \"$(string(eq))\". Since the flag @require_declaration is declared, all symbolic variables must be explicitly declared with the @species, @variables, and @parameters options."))
     end
 
-    return vars_extracted, add_default_diff, equations
+    return collect(vars_extracted), add_default_diff, equations
+end
+
+# Searches an expresion `expr` and returns true if it have any subexpression `D(...)` (where `...` can be anything).
+# Used to determine whether the default differential D has been used in any equation provided to `@equations`.
+function find_D_call(expr)
+    return if Base.isexpr(expr, :call) && expr.args[1] == :D
+        true
+    elseif expr isa Expr
+        any(find_D_call, expr.args)
+    else
+        false
+    end
 end
 
 # Creates an expression declaring differentials. Here, `tiv` is the time independent variables,
diff --git a/src/reactionsystem.jl b/src/reactionsystem.jl
index 83ed069ea0..a2cd0ecf2e 100644
--- a/src/reactionsystem.jl
+++ b/src/reactionsystem.jl
@@ -97,9 +97,9 @@ Base.@kwdef mutable struct NetworkProperties{I <: Integer, V <: BasicSymbolic{Re
     stronglinkageclasses::Vector{Vector{Int}} = Vector{Vector{Int}}(undef, 0)
     terminallinkageclasses::Vector{Vector{Int}} = Vector{Vector{Int}}(undef, 0)
 
-    checkedrobust::Bool = false 
+    checkedrobust::Bool = false
     robustspecies::Vector{Int} = Vector{Int}(undef, 0)
-    deficiency::Int = -1 
+    deficiency::Int = -1
 end
 #! format: on
 
@@ -215,11 +215,11 @@ end
 
 ### ReactionSystem Structure ###
 
-""" 
+"""
 WARNING!!!
 
-The following variable is used to check that code that should be updated when the `ReactionSystem` 
-fields are updated has in fact been updated. Do not just blindly update this without first checking 
+The following variable is used to check that code that should be updated when the `ReactionSystem`
+fields are updated has in fact been updated. Do not just blindly update this without first checking
 all such code and updating it appropriately (e.g. serialization). Please use a search for
 `reactionsystem_fields` throughout the package to ensure all places which should be updated, are updated.
 """
@@ -318,7 +318,7 @@ struct ReactionSystem{V <: NetworkProperties} <:
     """
     discrete_events::Vector{MT.SymbolicDiscreteCallback}
     """
-    Metadata for the system, to be used by downstream packages. 
+    Metadata for the system, to be used by downstream packages.
     """
     metadata::Any
     """
@@ -480,10 +480,10 @@ function ReactionSystem(iv; kwargs...)
     ReactionSystem(Reaction[], iv, [], []; kwargs...)
 end
 
-# Called internally (whether DSL-based or programmatic model creation is used). 
+# Called internally (whether DSL-based or programmatic model creation is used).
 # Creates a sorted reactions + equations vector, also ensuring reaction is first in this vector.
-# Extracts potential species, variables, and parameters from the input (if not provided as part of 
-# the model creation) and creates the corresponding vectors. 
+# Extracts potential species, variables, and parameters from the input (if not provided as part of
+# the model creation) and creates the corresponding vectors.
 # While species are ordered before variables in the unknowns vector, this ordering is not imposed here,
 # but carried out at a later stage.
 function make_ReactionSystem_internal(rxs_and_eqs::Vector, iv, us_in, ps_in;
@@ -495,7 +495,7 @@ function make_ReactionSystem_internal(rxs_and_eqs::Vector, iv, us_in, ps_in;
     any(in(obs_vars), us_in) &&
         error("Found an observable in the list of unknowns. This is not allowed.")
 
-    # Creates a combined iv vector (iv and sivs). This is used later in the function (so that 
+    # Creates a combined iv vector (iv and sivs). This is used later in the function (so that
     # independent variables can be excluded when encountered quantities are added to `us` and `ps`).
     t = value(iv)
     ivs = Set([t])
@@ -560,7 +560,7 @@ function make_ReactionSystem_internal(rxs_and_eqs::Vector, iv, us_in, ps_in;
     end
     psv = collect(new_ps)
 
-    # Passes the processed input into the next `ReactionSystem` call.    
+    # Passes the processed input into the next `ReactionSystem` call.
     ReactionSystem(fulleqs, t, usv, psv; spatial_ivs, continuous_events,
         discrete_events, observed, kwargs...)
 end
@@ -1062,8 +1062,8 @@ end
 
 ### General `ReactionSystem`-specific Functions ###
 
-# Checks if the `ReactionSystem` structure have been updated without also updating the 
-# `reactionsystem_fields` constant. If this is the case, returns `false`. This is used in 
+# Checks if the `ReactionSystem` structure have been updated without also updating the
+# `reactionsystem_fields` constant. If this is the case, returns `false`. This is used in
 # certain functionalities which would break if the `ReactionSystem` structure is updated without
 # also updating these functionalities.
 function reactionsystem_uptodate_check()
@@ -1241,7 +1241,7 @@ end
 ### `ReactionSystem` Remaking ###
 
 """
-    remake_ReactionSystem_internal(rs::ReactionSystem; 
+    remake_ReactionSystem_internal(rs::ReactionSystem;
         default_reaction_metadata::Vector{Pair{Symbol, T}} = Vector{Pair{Symbol, Any}}()) where {T}
 
 Takes a `ReactionSystem` and remakes it, returning a modified `ReactionSystem`. Modifications depend
@@ -1274,7 +1274,7 @@ function set_default_metadata(rs::ReactionSystem; default_reaction_metadata = []
     # Currently, `noise_scaling` is the only relevant metadata supported this way.
     drm_dict = Dict(default_reaction_metadata)
     if haskey(drm_dict, :noise_scaling)
-        # Finds parameters, species, and variables in the noise scaling term.       
+        # Finds parameters, species, and variables in the noise scaling term.
         ns_expr = drm_dict[:noise_scaling]
         ns_syms = [Symbolics.unwrap(sym) for sym in get_variables(ns_expr)]
         ns_ps = Iterators.filter(ModelingToolkit.isparameter, ns_syms)
@@ -1414,7 +1414,7 @@ function ModelingToolkit.compose(sys::ReactionSystem, systems::AbstractArray; na
         MT.collect_scoped_vars!(newunknowns, newparams, ssys, iv)
     end
 
-    if !isempty(newunknowns) 
+    if !isempty(newunknowns)
         @set! sys.unknowns = union(get_unknowns(sys), newunknowns)
         sort!(get_unknowns(sys), by = !isspecies)
         @set! sys.species = filter(isspecies, get_unknowns(sys))
diff --git a/test/dsl/dsl_options.jl b/test/dsl/dsl_options.jl
index e07f0cdbc3..16ef93d1f7 100644
--- a/test/dsl/dsl_options.jl
+++ b/test/dsl/dsl_options.jl
@@ -424,6 +424,74 @@ let
     end
 end
 
+# Tests that explicitly declaring a single symbol as several things does not work.
+# Several of these are broken, but note sure how to test broken-ness on `@test_throws false Exception @eval`.
+# Relevant issue: https://github.com/SciML/Catalyst.jl/issues/1173
+let
+    # Species + parameter.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species X(t)
+        #@parameters X
+    #end
+
+    # Species + variable.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species X(t)
+        #@variables X(t)
+    #end
+
+    # Variable + parameter.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@variables X(t)
+        #@parameters X
+    #end
+
+    # Species + differential.
+    @test_throws Exception @eval @reaction_network begin
+        @species X(t)
+        @differentials X = Differential(t)
+    end
+
+    # Parameter + differential.
+    @test_throws Exception @eval @reaction_network begin
+        @parameters X
+        @differentials X = Differential(t)
+    end
+
+    # Variable + differential.
+    @test_throws Exception @eval @reaction_network begin
+        @variables X(t)
+        @differentials X = Differential(t)
+    end
+
+    # Parameter + observable (species/variable + observable is OK, as this e.g. provide additional observables information).
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species Y(t)
+        #@parameters X
+        #@observables X ~ Y
+    #end
+
+    # Species + compound.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species X(t) O(t)
+        #@compounds begin X(t) ~ 2O end
+    #end
+
+    # Parameter + compound.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species O(t)
+        #@parameters X
+        #@compounds begin X(t) ~ 2O end
+    #end
+
+    # Variable + compound.
+    @test_broken false #@test_throws Exception @eval @reaction_network begin
+        #@species O(t)
+        #@variables X(t)
+        #@compounds begin X(t) ~ 2O end
+    #end
+end
+
 ### Test Independent Variable Designations ###
 
 # Test ivs in DSL.
@@ -450,6 +518,118 @@ let
     @test issetequal(Catalyst.get_sivs(rn), [x])
 end
 
+### Test Symbolic Variable Inference ###
+
+# Basic checks that that symbolic variables not explicitly declared are correctly inferred.
+let
+    # Case 1 (a reaction only).
+    rn1 = @reaction_network begin
+        (p1/(S1+p2) + S2), S1 --> S2
+    end
+    @test issetequal(species(rn1), [rn1.S1, rn1.S2])
+    @test issetequal(parameters(rn1), [rn1.p1, rn1.p2])
+
+    # Case 2 (reactions and equations).
+    rn2 = @reaction_network begin
+        @equations V1 + log(V2 + S1) ~ V2^2
+        (p1/V1 + S1 + log(S2 + V2 + p2)), S1 --> S2
+    end
+    @test issetequal(species(rn2), [rn2.S1, rn2.S2])
+    @test issetequal(nonspecies(rn2), [rn2.V1, rn2.V2])
+    @test issetequal(parameters(rn2), [rn2.p1, rn2.p2])
+
+    # Case 3 (reaction and equations with a differential).
+    rn3 = @reaction_network begin
+        @equations begin
+            D(V1) ~ S1 + V1
+            V2 + S2 ~ V1^2 + V2^2
+        end
+        (p1/V1 + S1 + log(S2 + V2 + p2)), S1 --> S2
+    end
+    @test issetequal(species(rn3), [rn3.S1, rn3.S2])
+    @test issetequal(nonspecies(rn3), [rn3.V1, rn3.V2])
+    @test issetequal(parameters(rn3), [rn3.p1, rn3.p2])
+
+    # Case 4 (reactions and equations with a pre-declared parameter).
+    rn4 = @reaction_network begin
+        @parameters p1
+        @equations V1 + sin(p1 + S1) ~ S2*V2
+        (p1+p2+V1+V2+S1+S2), S1 --> S2
+    end
+    @test issetequal(species(rn4), [rn2.S1, rn2.S2])
+    @test issetequal(nonspecies(rn4), [rn2.V1, rn2.V2])
+    @test issetequal(parameters(rn4), [rn2.p1, rn2.p2])
+
+    # Case 5 (algebraic equation containing D, which is pre-declared as a species).
+    rn5 = @reaction_network begin
+        @species D(t)
+        @equations D * (S1 + V1 + V2) ~ S2
+        (p1 + p2*(D + V1 + V2 + S2 + S2)), S1 --> S2 + D
+    end
+    @test issetequal(species(rn5), [rn5.S1, rn5.S2, rn5.D])
+    @test issetequal(nonspecies(rn5), [rn5.V1, rn5.V2])
+    @test issetequal(parameters(rn5), [rn5.p1, rn5.p2])
+
+    # Case 6 (algebraic equation containing D, which is pre-declared as a parameter).
+    rn6 = @reaction_network begin
+        @parameters D
+        @equations D * (S1 + V1 + V2) ~ S2
+        (p1 + p2*(D + V1 + V2 + S2 + S2)), S1 --> S2
+    end
+    @test issetequal(species(rn6), [rn6.S1, rn6.S2])
+    @test issetequal(nonspecies(rn6), [rn6.V1, rn6.V2])
+    @test issetequal(parameters(rn6), [rn6.p1, rn6.p2, rn6.D])
+
+    # Case 7 (algebraic equation containing D, which is pre-declared as a variable).
+    rn7 = @reaction_network begin
+        @variables D(t)
+        @equations D * (S1 + V1 + V2) ~ S2
+        (p1 + p2*(D + V1 + V2 + S2 + S2)), S1 --> S2
+    end
+    @test issetequal(species(rn7), [rn7.S1, rn7.S2])
+    @test issetequal(nonspecies(rn7), [rn7.V1, rn7.V2, rn7.D])
+    @test issetequal(parameters(rn7), [rn7.p1, rn7.p2])
+
+    # Case 8 (reactions, equations, and a custom differential).
+    rn8 = @reaction_network begin
+        @differentials Δ = Differential(t)
+        @equations Δ(V1) + Δ(V2) + log(V2 + S1) ~  S2
+        (p1/V1 + S1 + log(S2 + V2 + p2)), S1 --> S2
+    end
+    @test issetequal(species(rn8), [rn8.S1, rn8.S2])
+    @test issetequal(nonspecies(rn8), [rn8.V1, rn8.V2])
+    @test issetequal(parameters(rn8), [rn8.p1, rn8.p2])
+end
+
+# Checks that various cases where symbolic variables cannot (or shouldn't) be inferred generate errors.
+let
+    # Species/variables/parameter named after default differential used as function call.
+    # In the future, species/variables should be usable this way (designating a time delay).
+    @test_throws Exception @eval @reaction_network begin
+        @equations D(V) ~ 1 - V
+        d, D --> 0
+    end
+    @test_broken false # @test_throws Exception @eval @reaction_network begin
+        #@variables D(t)
+        #@equations D(V) ~ 1 - V
+        #d, X --> 0
+    #end
+    @test_throws Exception @eval @reaction_network begin
+        @parameters D
+        @equations D(V) ~ 1 - V
+        d, X --> 0
+    end
+
+    # Symbol only occurring in events.
+    @test_throws Exception @eval @reaction_network begin
+        @discrete_event (X > 1.0) => [V => V/2]
+        d, X --> 0
+    end
+    @test_throws Exception @eval @reaction_network begin
+        @continuous_event [X > 1.0] => [V => V/2]
+        d, X --> 0
+    end
+end
 
 ### Observables ###
 
@@ -790,7 +970,7 @@ end
 # Check that DAE is solved correctly.
 let
     rn = @reaction_network rn begin
-        @parameters k
+        @parameters k d
         @variables X(t) Y(t)
         @equations begin
             X + 5 ~ k*S
@@ -948,12 +1128,6 @@ let
         @equations X = 1 - S
         (p,d), 0 <--> S
     end
-
-    # Equation with component undeclared elsewhere.
-    @test_throws Exception @eval @reaction_network begin
-        @equations X ~ p - S
-        (P,D), 0 <--> S
-    end
 end
 
 # test combinatoric_ratelaws DSL option
@@ -1062,7 +1236,7 @@ let
     @test isequal(Catalyst.expand_registered_functions(equations(rn4)[1]), D(A) ~ v*(A^n))
 end
 
-### test that @no_infer properly throws errors when undeclared variables are written
+### test that @no_infer properly throws errors when undeclared variables are written ###
 
 import Catalyst: UndeclaredSymbolicError
 let
@@ -1110,12 +1284,12 @@ let
     # Test error when a variable in an equation is inferred
     @test_throws UndeclaredSymbolicError @macroexpand @reaction_network begin
         @require_declaration
-        @equations D(V) ~ V^2
+        @equations V ~ V^2 + 2
     end
     @test_nowarn @macroexpand @reaction_network begin
         @require_declaration
         @variables V(t)
-        @equations D(V) ~ V^2
+        @equations V ~ V^2 + 2
     end
 
     # Test error when a variable in an observable is inferred
@@ -1129,4 +1303,16 @@ let
         @variables X1(t) X2(t)
         @observables X2 ~ X1
     end
+
+    # Test when the default differential D is inferred
+    @test_throws UndeclaredSymbolicError @macroexpand @reaction_network begin
+        @require_declaration
+        @variables V(t)
+        @equations D(V) ~ 1 - V
+    end
+    @test_nowarn @macroexpand @reaction_network begin
+        @differentials D = Differential(t)
+        @variables X1(t) X2(t)
+        @observables X2 ~ X1
+    end
 end
diff --git a/test/reactionsystem_core/coupled_equation_crn_systems.jl b/test/reactionsystem_core/coupled_equation_crn_systems.jl
index 3b4921ff96..8f631d58d3 100644
--- a/test/reactionsystem_core/coupled_equation_crn_systems.jl
+++ b/test/reactionsystem_core/coupled_equation_crn_systems.jl
@@ -899,38 +899,6 @@ end
 
 # Checks that various misformatted declarations yield errors.
 let
-    # Symbol in equation not appearing elsewhere (1).
-    @test_throws Exception @eval @reaction_network begin
-        @equations D(V) ~ -X
-    end
-
-    # Symbol in equation not appearing elsewhere (2).
-    @test_throws Exception @eval @reaction_network begin
-        @equations 1 + log(x) ~ 2X
-    end
-
-    # Attempting to infer differential variable not isolated on lhs (1).
-    @test_throws Exception @eval @reaction_network begin
-        @equations D(V) + 1 ~ 0
-    end
-
-    # Attempting to infer differential variable not isolated on lhs (2).
-    @test_throws Exception @eval @reaction_network begin
-        @equations -1.0 ~ D(V)
-    end
-
-    # Attempting to infer differential operator not isolated on lhs (1).
-    @test_throws Exception @eval @reaction_network begin
-        @variables V(t)
-        @equations D(V) + 1 ~ 0
-    end
-
-    # Attempting to infer a variable when using a non-default differential.
-    @test_throws Exception @eval @reaction_network begin
-        @differentials Δ = Differential(t)
-        @equations Δ(V) ~ -1,0
-    end
-
     # Attempting to create a new differential from an unknown iv.
     @test_throws Exception @eval @reaction_network begin
         @differentials D = Differential(τ)
@@ -944,14 +912,8 @@ let
 
     # Several equations without `begin ... end` block.
     @test_throws Exception @eval @reaction_network begin
-        @variables V(t)
         @equations D(V) + 1 ~ - 1.0
-    end
-
-    # Undeclared differential.
-    @test_throws Exception @eval @reaction_network begin
-        @species V
-        @equations Δ(V) ~ -1.0
+        @equations D(W) + 1 ~ - 1.0
     end
 
     # System using multiple ivs.