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 00:45:13 2015 +0200
@@ -2699,6 +2699,23 @@ class UnicodeTest(string_tests.CommonTes
         self.assertTrue(astral >= bmp2)
         self.assertFalse(astral >= astral2)
 
+    def test_pep393_utf8_caching_bug(self):
+        # Issue #25709: Problem with string concatenation and utf-8 cache
+        for k in 0x24, 0xa4, 0x20ac, 0x1f40d:
+            t = ''
+            for i in range(5):
+                t += chr(k)
+                # The test has to call PyUnicode_AsUTF8AndSize() to create
+                # the UTF-8 encoded string cached in the Unicode object.
+                # Other ways are: float, complex, compile...
+                try:
+                    int(t)
+                except ValueError:
+                    pass
+                # t.encode() calls _PyUnicode_AsUTF8String() to retrieve
+                # the cached UTF-8 encode string.
+                self.assertEqual(t.encode().decode(), t)
+
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):
@@ -2750,6 +2767,5 @@ 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 00:45:13 2015 +0200
@@ -907,6 +907,11 @@ resize_compact(PyObject *unicode, Py_ssi
         PyObject_DEL(_PyUnicode_WSTR(unicode));
         _PyUnicode_WSTR(unicode) = NULL;
     }
+    if (_PyUnicode_HAS_UTF8_MEMORY(unicode)) {
+        PyObject_DEL(_PyUnicode_UTF8(unicode));
+        _PyUnicode_UTF8(unicode) = NULL;
+        _PyUnicode_UTF8_LENGTH(unicode) = 0;
+    }
 #ifdef Py_DEBUG
     unicode_fill_invalid(unicode, old_length);
 #endif