diff --git a/twisted/python/test/test_util.py b/twisted/python/test/test_util.py
index e36ef2b..19b79cd 100644
--- a/twisted/python/test/test_util.py
+++ b/twisted/python/test/test_util.py
@@ -895,14 +895,31 @@ class SlowStringCompareTests(unittest.TestCase):
         C{unicode} object.
         """
         def _compare(s1, s2):
-            expected = s1 == s2
+            if sys.version_info >= (2, 5):
+                expected = s1 == s2
+
+                ws = self.flushWarnings(
+                    [SlowStringCompareTests.test_unicodeComparison])
+                for w in ws:
+                    self.assertEquals(w['category'], UnicodeWarning)
+
+                result = util.slowStringCompare(s1, s2)
+            else:
+                # When Python 2.4 cannot decode the non-unicode side of a string
+                # comparion, it raises UnicodeDecodeError instead of giving a
+                # UnicodeWarning and returning False.
+                try:
+                    expected = s1 == s2
+                except UnicodeDecodeError:
+                    # Use the exception class itself as a placeholder to represent
+                    # the raising of the exception.
+                    expected = UnicodeDecodeError
+
+                try:
+                    result = util.slowStringCompare(s1, s2)
+                except UnicodeDecodeError:
+                    result = UnicodeDecodeError
 
-            ws = self.flushWarnings(
-                [SlowStringCompareTests.test_unicodeComparison])
-            for w in ws:
-                self.assertEquals(w['category'], UnicodeWarning)
-
-            result = util.slowStringCompare(s1, s2)
             self.assertEquals(result, expected)
 
             [w] = self.flushWarnings(
diff --git a/twisted/python/util.py b/twisted/python/util.py
index 0e9cc12..e15ef89 100644
--- a/twisted/python/util.py
+++ b/twisted/python/util.py
@@ -1001,11 +1001,19 @@ def slowStringCompare(s1, s2):
         try:
             s2 = unicode(s2)
         except UnicodeDecodeError:
+            if sys.version_info < (2, 5):
+                # When Python 2.4 cannot decode the non-unicode side of a
+                # string comparion, it raises UnicodeDecodeError instead of
+                # giving a UnicodeWarning and returning False.  Match this
+                # behavior and re-raise the exception.
+                raise
             return False
     elif not isinstance(s1, unicode) and isinstance(s2, unicode):
         try:
             s1 = unicode(s1)
         except UnicodeDecodeError:
+            if sys.version_info < (2, 5):
+                raise
             return False
 
     if len(s1) != len(s2):