diff --git a/src/cogpowered/FineDiff/Parser/Operations/Insert.php b/src/cogpowered/FineDiff/Parser/Operations/Insert.php
index 64a73c2..efa2631 100644
--- a/src/cogpowered/FineDiff/Parser/Operations/Insert.php
+++ b/src/cogpowered/FineDiff/Parser/Operations/Insert.php
@@ -46,7 +46,7 @@ public function getFromLen()
      */
     public function getToLen()
     {
-        return strlen($this->text);
+        return mb_strlen($this->text);
     }
 
     /**
@@ -62,7 +62,7 @@ public function getText()
      */
     public function getOpcode()
     {
-        $to_len = strlen($this->text);
+        $to_len = mb_strlen($this->text);
 
         if ( $to_len === 1 ) {
             return "i:{$this->text}";
diff --git a/src/cogpowered/FineDiff/Parser/Operations/Replace.php b/src/cogpowered/FineDiff/Parser/Operations/Replace.php
index 515af13..fe5f3b3 100644
--- a/src/cogpowered/FineDiff/Parser/Operations/Replace.php
+++ b/src/cogpowered/FineDiff/Parser/Operations/Replace.php
@@ -43,7 +43,7 @@ public function getFromLen()
      */
     public function getToLen()
     {
-        return strlen($this->text);
+        return mb_strlen($this->text);
     }
 
     /**
@@ -67,7 +67,7 @@ public function getOpcode()
             $del_opcode = "d{$this->fromLen}";
         }
 
-        $to_len = strlen($this->text);
+        $to_len = mb_strlen($this->text);
 
         if ($to_len === 1) {
             return "{$del_opcode}i:{$this->text}";
diff --git a/src/cogpowered/FineDiff/Parser/Parser.php b/src/cogpowered/FineDiff/Parser/Parser.php
index b85415b..10dbb87 100644
--- a/src/cogpowered/FineDiff/Parser/Parser.php
+++ b/src/cogpowered/FineDiff/Parser/Parser.php
@@ -155,7 +155,7 @@ protected function process($from_text, $to_text)
             // increase granularity
             if ($fragment instanceof Replace && $has_next_stage) {
                 $this->process(
-                    substr($this->from_text, $this->from_offset, $fragment->getFromLen()),
+                    mb_substr($this->from_text, $this->from_offset, $fragment->getFromLen()),
                     $fragment->getText()
                 );
             }
@@ -195,8 +195,8 @@ protected function diff($from_text, $to_text, $delimiters)
         $result = array();
 
         // fragment-level diffing
-        $from_text_len  = strlen($from_text);
-        $to_text_len    = strlen($to_text);
+        $from_text_len  = mb_strlen($from_text);
+        $to_text_len    = mb_strlen($to_text);
         $from_fragments = $this->extractFragments($from_text, $delimiters);
         $to_fragments   = $this->extractFragments($to_text, $delimiters);
 
@@ -218,7 +218,7 @@ protected function diff($from_text, $to_text, $delimiters)
                 if ( $from_segment_length ) {
                     $result[$from_segment_start * 4] = new Delete($from_segment_length);
                 } else if ( $to_segment_length ) {
-                    $result[$from_segment_start * 4 + 1] = new Insert(substr($to_text, $to_segment_start, $to_segment_length));
+                    $result[$from_segment_start * 4 + 1] = new Insert(mb_substr($to_text, $to_segment_start, $to_segment_length));
                 }
 
                 continue;
@@ -233,7 +233,7 @@ protected function diff($from_text, $to_text, $delimiters)
             while ( $from_base_fragment_index < $from_segment_end ) {
 
                 $from_base_fragment        = $from_fragments[$from_base_fragment_index];
-                $from_base_fragment_length = strlen($from_base_fragment);
+                $from_base_fragment_length = mb_strlen($from_base_fragment);
 
                 // performance boost: cache array keys
                 if (!isset($cached_array_keys_for_current_segment[$from_base_fragment])) {
@@ -297,7 +297,7 @@ protected function diff($from_text, $to_text, $delimiters)
                             break;
                         }
 
-                        $fragment_length = strlen($from_fragments[$fragment_from_index]);
+                        $fragment_length = mb_strlen($from_fragments[$fragment_from_index]);
                         $fragment_index_offset += $fragment_length;
                     }
 
@@ -308,7 +308,7 @@ protected function diff($from_text, $to_text, $delimiters)
                     }
                 }
 
-                $from_base_fragment_index += strlen($from_base_fragment);
+                $from_base_fragment_index += mb_strlen($from_base_fragment);
 
                 // If match is larger than half segment size, no point trying to find better
                 // TODO: Really?
@@ -328,7 +328,7 @@ protected function diff($from_text, $to_text, $delimiters)
                 $result[$best_from_start * 4 + 2] = new Copy($best_copy_length);
                 $jobs[] = array($best_from_start + $best_copy_length, $from_segment_end, $best_to_start + $best_copy_length, $to_segment_end);
             } else {
-                $result[$from_segment_start * 4 ] = new Replace($from_segment_length, substr($to_text, $to_segment_start, $to_segment_length));
+                $result[$from_segment_start * 4 ] = new Replace($from_segment_length, mb_substr($to_text, $to_segment_start, $to_segment_length));
             }
         }
 
@@ -346,7 +346,7 @@ protected function diff($from_text, $to_text, $delimiters)
     protected function charDiff($from_text, $to_text)
     {
         $result = array();
-        $jobs   = array(array(0, strlen($from_text), 0, strlen($to_text)));
+        $jobs   = array(array(0, mb_strlen($from_text), 0, mb_strlen($to_text)));
 
         while ($job = array_pop($jobs)) {
 
@@ -362,7 +362,7 @@ protected function charDiff($from_text, $to_text)
                 if ($from_segment_len) {
                     $result[$from_segment_start * 4 + 0] = new Delete($from_segment_len);
                 } else if ( $to_segment_len ) {
-                    $result[$from_segment_start * 4 + 1] = new Insert(substr($to_text, $to_segment_start, $to_segment_len));
+                    $result[$from_segment_start * 4 + 1] = new Insert(mb_substr($to_text, $to_segment_start, $to_segment_len));
                 }
 
                 continue;
@@ -379,7 +379,7 @@ protected function charDiff($from_text, $to_text)
 
                     while ($to_copy_start <= $to_copy_start_max) {
 
-                        $from_copy_start = strpos(substr($from_text, $from_segment_start, $from_segment_len), substr($to_text, $to_copy_start, $copy_len));
+                        $from_copy_start = mb_strpos(mb_substr($from_text, $from_segment_start, $from_segment_len), mb_substr($to_text, $to_copy_start, $copy_len));
 
                         if ($from_copy_start !== false) {
                             $from_copy_start += $from_segment_start;
@@ -402,7 +402,7 @@ protected function charDiff($from_text, $to_text)
 
                     while ($from_copy_start <= $from_copy_start_max) {
 
-                        $to_copy_start = strpos(substr($to_text, $to_segment_start, $to_segment_len), substr($from_text, $from_copy_start, $copy_len));
+                        $to_copy_start = mb_strpos(mb_substr($to_text, $to_segment_start, $to_segment_len), mb_substr($from_text, $from_copy_start, $copy_len));
 
                         if ($to_copy_start !== false) {
                             $to_copy_start += $to_segment_start;
@@ -424,7 +424,7 @@ protected function charDiff($from_text, $to_text)
             }
             // no match,  so delete all, insert all
             else {
-                $result[$from_segment_start * 4] = new Replace($from_segment_len, substr($to_text, $to_segment_start, $to_segment_len));
+                $result[$from_segment_start * 4] = new Replace($from_segment_len, mb_substr($to_text, $to_segment_start, $to_segment_len));
             }
         }
 
@@ -448,7 +448,7 @@ protected function extractFragments($text, $delimiters)
         // special case: split into characters
         if (empty($delimiters)) {
             $chars                = str_split($text, 1);
-            $chars[strlen($text)] = '';
+            $chars[mb_strlen($text)] = '';
 
             return $chars;
         }
@@ -466,7 +466,7 @@ protected function extractFragments($text, $delimiters)
                 break;
             }
 
-            $fragments[$start] = substr($text, $start, $end - $start);
+            $fragments[$start] = mb_substr($text, $start, $end - $start);
             $start             = $end;
         }
 
diff --git a/src/cogpowered/FineDiff/Render/Html.php b/src/cogpowered/FineDiff/Render/Html.php
index 3788598..7060ee7 100644
--- a/src/cogpowered/FineDiff/Render/Html.php
+++ b/src/cogpowered/FineDiff/Render/Html.php
@@ -25,10 +25,10 @@ class Html extends Renderer
     public function callback($opcode, $from, $from_offset, $from_len)
     {
         if ($opcode === 'c') {
-            $html = htmlentities(substr($from, $from_offset, $from_len));
+            $html = htmlentities(mb_substr($from, $from_offset, $from_len));
         } else if ($opcode === 'd') {
 
-            $deletion = substr($from, $from_offset, $from_len);
+            $deletion = mb_substr($from, $from_offset, $from_len);
 
             if (strcspn($deletion, " \n\r") === 0) {
                 $deletion = str_replace(array("\n","\r"), array('\n','\r'), $deletion);
@@ -37,7 +37,7 @@ public function callback($opcode, $from, $from_offset, $from_len)
             $html = '<del>'.htmlentities($deletion).'</del>';
 
         } else /* if ( $opcode === 'i' ) */ {
-            $html = '<ins>'.htmlentities(substr($from, $from_offset, $from_len)).'</ins>';
+            $html = '<ins>'.htmlentities(mb_substr($from, $from_offset, $from_len)).'</ins>';
         }
 
         return $html;
diff --git a/src/cogpowered/FineDiff/Render/Renderer.php b/src/cogpowered/FineDiff/Render/Renderer.php
index 7d848ea..fd4efa5 100644
--- a/src/cogpowered/FineDiff/Render/Renderer.php
+++ b/src/cogpowered/FineDiff/Render/Renderer.php
@@ -43,18 +43,18 @@ public function process($from_text, $opcodes)
         // Holds the generated string that is returned
         $output = '';
 
-        $opcodes_len    = strlen($opcodes);
+        $opcodes_len    = mb_strlen($opcodes);
         $from_offset    = 0;
         $opcodes_offset = 0;
 
         while ($opcodes_offset < $opcodes_len) {
 
-            $opcode = substr($opcodes, $opcodes_offset, 1);
+            $opcode = mb_substr($opcodes, $opcodes_offset, 1);
             $opcodes_offset++;
-            $n = intval(substr($opcodes, $opcodes_offset));
+            $n = intval(mb_substr($opcodes, $opcodes_offset));
 
             if ($n) {
-                $opcodes_offset += strlen(strval($n));
+                $opcodes_offset += mb_strlen(strval($n));
             } else {
                 $n = 1;
             }
diff --git a/src/cogpowered/FineDiff/Render/Text.php b/src/cogpowered/FineDiff/Render/Text.php
index 699e864..42be0d0 100644
--- a/src/cogpowered/FineDiff/Render/Text.php
+++ b/src/cogpowered/FineDiff/Render/Text.php
@@ -23,7 +23,7 @@ class Text extends Renderer
     public function callback($opcode, $from, $from_offset, $from_len)
     {
         if ($opcode === 'c' || $opcode === 'i') {
-            return substr($from, $from_offset, $from_len);
+            return mb_substr($from, $from_offset, $from_len);
         }
 
         return '';