diff -r ebec1a98ab81 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Mon Nov 23 16:44:30 2015 +0200
+++ b/Lib/test/test_unicode.py	Tue Nov 24 10:48:00 2015 +0200
@@ -2699,6 +2699,20 @@ class UnicodeTest(string_tests.CommonTes
         self.assertTrue(astral >= bmp2)
         self.assertFalse(astral >= astral2)
 
+    @support.cpython_only
+    def test_pep393_utf8_caching_bug(self):
+        # Issue #25709: Problem with string concatenation and utf-8 cache
+        from _testcapi import getargs_s_hash
+        for k in 0x24, 0xa4, 0x20ac, 0x1f40d:
+            s = ''
+            for i in range(5):
+                s += chr(k)
+                # Parsing with the "s#" format code calls indirectly
+                # PyUnicode_AsUTF8AndSize() which creates the UTF-8
+                # encoded string cached in the Unicode object.
+                res = getargs_s_hash(s)
+                self.assertEqual(res, chr(k).encode() * (i + 1))
+
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):
@@ -2750,6 +2764,5 @@ class StringModuleTest(unittest.TestCase
             ]])
         self.assertRaises(TypeError, _string.formatter_field_name_split, 1)
 
-
 if __name__ == "__main__":
     unittest.main()
diff -r ebec1a98ab81 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Mon Nov 23 16:44:30 2015 +0200
+++ b/Objects/unicodeobject.c	Tue Nov 24 10:48:00 2015 +0200
@@ -885,6 +885,11 @@ resize_compact(PyObject *unicode, Py_ssi
     }
     new_size = (struct_size + (length + 1) * char_size);
 
+    if (_PyUnicode_HAS_UTF8_MEMORY(unicode)) {
+        PyObject_DEL(_PyUnicode_UTF8(unicode));
+        _PyUnicode_UTF8(unicode) = NULL;
+        _PyUnicode_UTF8_LENGTH(unicode) = 0;
+    }
     _Py_DEC_REFTOTAL;
     _Py_ForgetReference(unicode);