diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -25,10 +25,16 @@ def search_function(encoding):
         return (42, 42) # no unicode
     def decode2(input, errors="strict"):
         return (42, 42) # no unicode
+    def encode3(input, errors="strict"):
+        return (u'42', 42) # not bytes
+    def decode3(input, errors="strict"):
+        return ('42', 42) # not unicode
     if encoding=="test.unicode1":
         return (encode1, decode1, None, None)
     elif encoding=="test.unicode2":
         return (encode2, decode2, None, None)
+    elif encoding=="test.unicode3":
+        return (encode3, decode3, None, None)
     else:
         return None
 codecs.register(search_function)
@@ -1851,6 +1857,27 @@ class UnicodeTest(
                     unicode_encodedecimal(u"123" + s, "xmlcharrefreplace"),
                     '123' + exp)
 
+    def test_py3k_warning_encode_not_bytes(self):
+        with test_support.check_py3k_warnings(
+                ('encoders must return bytestrings in 3.x', DeprecationWarning)):
+            u"abcd".encode('test.unicode3')
+
+    def test_py3k_warning_decode_not_unicode(self):
+        with test_support.check_py3k_warnings(
+                ('decoders must return unicode in 3.x', DeprecationWarning)):
+            "abcd".decode('test.unicode3')
+
+    def test_py3k_warning_str_encode(self):
+        with test_support.check_py3k_warnings(
+                (r'bytes.encode\(\) does not exist in 3.x', DeprecationWarning)):
+            "abcd".encode('ascii')
+
+    def test_py3k_warning_unicode_decode(self):
+        with test_support.check_py3k_warnings(
+                (r'unicode.decode\(\) does not exist in 3.x', DeprecationWarning)):
+            u"abcd".decode('ascii')
+
+
 def test_main():
     test_support.run_unittest(__name__)
 
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -3010,6 +3010,9 @@ string_encode(PyStringObject *self, PyOb
     char *errors = NULL;
     PyObject *v;
 
+    if (PyErr_WarnPy3k("bytes.encode() does not exist in 3.x", 1) < 0)
+        return NULL;
+
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|ss:encode",
                                      kwlist, &encoding, &errors))
         return NULL;
@@ -3063,6 +3066,13 @@ string_decode(PyStringObject *self, PyOb
         Py_DECREF(v);
         return NULL;
     }
+    if (!PyUnicode_Check(v)) {
+        if (PyErr_WarnPy3k("decoders must return unicode in 3.x; "
+                           "use codecs.decode() to decode to arbitrary types", 1) < 0) {
+            Py_DECREF(v);
+            return NULL;
+        }
+    }
     return v;
 
  onError:
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -6451,6 +6451,13 @@ unicode_encode(PyUnicodeObject *self, Py
         Py_DECREF(v);
         return NULL;
     }
+    if (!PyBytes_Check(v)) {
+        if (PyErr_WarnPy3k("encoders must return bytestrings in 3.x; "
+                           "use codecs.encode() to encode to arbitrary types", 1) < 0) {
+            Py_DECREF(v);
+            return NULL;
+        }
+    }
     return v;
 
   onError:
@@ -6475,6 +6482,9 @@ unicode_decode(PyUnicodeObject *self, Py
     char *errors = NULL;
     PyObject *v;
 
+    if (PyErr_WarnPy3k("unicode.decode() does not exist in 3.x", 1) < 0)
+        return NULL;
+
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|ss:decode",
                                      kwlist, &encoding, &errors))
         return NULL;