diff --git a/Include/stringobject.h b/Include/stringobject.h
index 12cc093c629e973..8232258b6c58cdc 100644
--- a/Include/stringobject.h
+++ b/Include/stringobject.h
@@ -36,6 +36,7 @@ typedef struct {
     PyObject_VAR_HEAD
     long ob_shash;
     int ob_sstate;
+    Py_ssize_t ob_bstate;
     char ob_sval[1];
 
     /* Invariants:
@@ -52,6 +53,10 @@ typedef struct {
 #define SSTATE_INTERNED_MORTAL 1
 #define SSTATE_INTERNED_IMMORTAL 2
 
+#define BSTATE_NOT_SURE 0
+#define BSTATE_BYTE 1
+#define BSTATE_UNICODE 2
+
 PyAPI_DATA(PyTypeObject) PyBaseString_Type;
 PyAPI_DATA(PyTypeObject) PyString_Type;
 
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 7781f9663015fbb..f6e46ac29073275 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -415,6 +415,7 @@ extern "C" {
 typedef struct {
     PyObject_HEAD
     Py_ssize_t length;          /* Length of raw Unicode data in buffer */
+    Py_ssize_t ob_bstate;
     Py_UNICODE *str;            /* Raw Unicode buffer */
     long hash;                  /* Hash value; -1 if not set */
     PyObject *defenc;           /* (Default) Encoded version as Python
@@ -422,6 +423,10 @@ typedef struct {
                                    implementing the buffer protocol */
 } PyUnicodeObject;
 
+#define BSTATE_NOT_SURE 0
+#define BSTATE_BYTE 1
+#define BSTATE_UNICODE 2
+
 PyAPI_DATA(PyTypeObject) PyUnicode_Type;
 
 #define PyUnicode_Check(op) \
diff --git a/Lib/test/test_py3kwarn.py b/Lib/test/test_py3kwarn.py
index efcd1d2f9d8447d..9ec40a0b64b6975 100644
--- a/Lib/test/test_py3kwarn.py
+++ b/Lib/test/test_py3kwarn.py
@@ -208,6 +208,22 @@ def set():
             with check_py3k_warnings() as w:
                 self.assertWarning(set(), w, expected)
 
+    def test_bytes_parsing(self):
+        with check_py3k_warnings():
+            b"{0}-{1}: {2}".format(1,"foo",True)
+            b"{0}-{1}: {2}".encode()
+
+    def test_str_parsing(self):
+        with check_py3k_warnings():
+            "{0}-{1}: {2}".decode()
+
+    def test_string_parsing(self):
+        with check_py3k_warnings():
+            b"{0}-{1}: {2}"._formatter_parser()
+            b"{0}-{1}: {2}"._formatter_field_name_split()
+            "{0}-{1}: {2}"._formatter_parser()
+            "{0}-{1}: {2}"._formatter_field_name_split()
+
     def test_slice_methods(self):
         class Spam(object):
             def __getslice__(self, i, j): pass
diff --git a/Objects/stringlib/string_format.h b/Objects/stringlib/string_format.h
index 2bd1839d72e73fb..c9f9f655ef2900a 100644
--- a/Objects/stringlib/string_format.h
+++ b/Objects/stringlib/string_format.h
@@ -1186,6 +1186,10 @@ formatter_parser(STRINGLIB_OBJECT *self)
 {
     formatteriterobject *it;
 
+    if (PyErr_WarnPy3k("'_format_parser()' is not supported for both unicode and bytes in 3.x: use alternate format parsing syntax.", 1) < 0) {
+        return NULL;
+    }
+
     it = PyObject_New(formatteriterobject, &PyFormatterIter_Type);
     if (it == NULL)
         return NULL;
@@ -1326,6 +1330,10 @@ formatter_field_name_split(STRINGLIB_OBJECT *self)
     PyObject *first_obj = NULL;
     PyObject *result = NULL;
 
+    if (PyErr_WarnPy3k("'_formatter_field_name_split()' is not supported for both unicode and bytes in 3.x: use alternate formatter split syntax.", 1) < 0) {
+        return NULL;
+    }
+
     it = PyObject_New(fieldnameiterobject, &PyFieldNameIter_Type);
     if (it == NULL)
         return NULL;
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index fd477627f6e62e9..d4b536565f760c5 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -3016,6 +3016,9 @@ string_encode(PyStringObject *self, PyObject *args, PyObject *kwargs)
     char *errors = NULL;
     PyObject *v;
 
+    if (PyErr_WarnPy3k("'encode()' is not supported on bytes in 3.x: convert the string to unicode.", 1) < 0) {
+        return NULL;
+    }
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|ss:encode",
                                      kwlist, &encoding, &errors))
         return NULL;
@@ -3055,6 +3058,15 @@ string_decode(PyStringObject *self, PyObject *args, PyObject *kwargs)
     char *errors = NULL;
     PyObject *v;
 
+    if (PyString_CheckExact(self)) {
+        self->ob_bstate = BSTATE_BYTE;
+    }
+
+    if ((self->ob_bstate == BSTATE_BYTE) &&
+        PyErr_WarnPy3k("'decode()' is not supported on 'str' in 3.x: convert the string to bytes.", 1) < 0) {
+        return NULL;
+    }
+
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|ss:decode",
                                      kwlist, &encoding, &errors))
         return NULL;
@@ -3610,6 +3622,10 @@ string__format__(PyObject* self, PyObject* args)
     PyObject *result = NULL;
     PyObject *tmp = NULL;
 
+    if (PyErr_WarnPy3k("'format()' is not supported for bytes in 3.x: use alternate format syntax.", 1) < 0) {
+        return NULL;
+    }
+
     /* If 2.x, convert format_spec to the same type as value */
     /* This is to allow things like u''.format('') */
     if (!PyArg_ParseTuple(args, "O:__format__", &format_spec))