diff -r ebec1a98ab81 Lib/test/pickletester.py
--- a/Lib/test/pickletester.py	Mon Nov 23 16:44:30 2015 +0200
+++ b/Lib/test/pickletester.py	Mon Nov 23 22:12:48 2015 +0200
@@ -1300,6 +1300,18 @@ class AbstractPickleTests(unittest.TestC
             t2 = self.loads(p)
             self.assert_is_copy(t, t2)
 
+    def test_pep393_utf8_caching_bug(self):
+        # Issue #25709: Problem with string concatenation and utf-8 cache
+        for proto in protocols:
+            with self.subTest(proto=proto):
+                for k in 0x24, 0xa4, 0x20ac, 0x1f40d:
+                    t = ''
+                    for i in range(5):
+                        t += chr(k)
+                        p = self.dumps(t, proto)
+                        t2 = self.loads(p)
+                        self.assert_is_copy(t, t2)
+
     def test_bytes(self):
         for proto in protocols:
             for s in b'', b'xyz', b'xyz'*100:
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	Mon Nov 23 22:12:48 2015 +0200
@@ -2699,6 +2699,20 @@ 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)
+                # Create UTF8 representation.
+                # Other ways are: float, complex, compile...
+                try:
+                    int(t)
+                except ValueError:
+                    pass
+                self.assertEqual(t.encode().decode(), t)
+
 
 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	Mon Nov 23 22:12:48 2015 +0200
@@ -862,7 +862,7 @@ resize_compact(PyObject *unicode, Py_ssi
     Py_ssize_t char_size;
     Py_ssize_t struct_size;
     Py_ssize_t new_size;
-    int share_wstr;
+    int share_wstr, share_utf8;
     PyObject *new_unicode;
 #ifdef Py_DEBUG
     Py_ssize_t old_length = _PyUnicode_LENGTH(unicode);
@@ -878,6 +878,7 @@ resize_compact(PyObject *unicode, Py_ssi
     else
         struct_size = sizeof(PyCompactUnicodeObject);
     share_wstr = _PyUnicode_SHARE_WSTR(unicode);
+    share_utf8 = _PyUnicode_SHARE_UTF8(unicode);
 
     if (length > ((PY_SSIZE_T_MAX - struct_size) / char_size - 1)) {
         PyErr_NoMemory();
@@ -907,6 +908,15 @@ resize_compact(PyObject *unicode, Py_ssi
         PyObject_DEL(_PyUnicode_WSTR(unicode));
         _PyUnicode_WSTR(unicode) = NULL;
     }
+    if (share_utf8) {
+        _PyUnicode_UTF8(unicode) = PyUnicode_DATA(unicode);
+        _PyUnicode_UTF8_LENGTH(unicode) = length;
+    }
+    else 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