diff --git a/tests/test_datastore.py b/tests/test_datastore.py
index 26a41bb7c1..fc624809a8 100644
--- a/tests/test_datastore.py
+++ b/tests/test_datastore.py
@@ -1877,6 +1877,47 @@ def testCacheExpiryDatasetsFromDisabled(self) -> None:
             self.assertExpiration(cache_manager, 5, threshold + 1)
             self.assertIn(f"{mode}={threshold}", str(cache_manager))
 
+    def testExpirationModeOverride(self) -> None:
+        threshold = 2  # Keep 2 datasets.
+        mode = "datasets"
+        config_str = self._expiration_config(mode, threshold)
+
+        mode = "size"
+        threshold = 55
+        with unittest.mock.patch.dict(
+            os.environ,
+            {"DAF_BUTLER_CACHE_EXPIRATION_MODE": f"{mode}={threshold}"},
+        ):
+            cache_manager = self._make_cache_manager(config_str)
+            self.assertExpiration(cache_manager, 10, 6)
+            self.assertIn(f"{mode}={threshold}", str(cache_manager))
+
+        # Check we get a warning with unrecognized form.
+        with unittest.mock.patch.dict(
+            os.environ,
+            {"DAF_BUTLER_CACHE_EXPIRATION_MODE": "something"},
+        ):
+            with self.assertLogs(level="WARNING") as cm:
+                self._make_cache_manager(config_str)
+            self.assertIn("Unrecognized form (something)", cm.output[0])
+
+        with unittest.mock.patch.dict(
+            os.environ,
+            {"DAF_BUTLER_CACHE_EXPIRATION_MODE": "something=5"},
+        ):
+            with self.assertRaises(ValueError) as cm:
+                self._make_cache_manager(config_str)
+            self.assertIn("Unrecognized value", str(cm.exception))
+
+    def testMissingThreshold(self) -> None:
+        threshold = ""
+        mode = "datasets"
+        config_str = self._expiration_config(mode, threshold)
+
+        with self.assertRaises(ValueError) as cm:
+            self._make_cache_manager(config_str)
+        self.assertIn("Cache expiration threshold", str(cm.exception))
+
     def testCacheExpiryDatasetsComposite(self) -> None:
         threshold = 2  # Keep 2 datasets.
         mode = "datasets"