diff --git a/Include/object.h b/Include/object.h
index 807b24188a75b2..43181a4b5fde9d 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -465,6 +465,7 @@ PyAPI_FUNC(PyObject *) PyObject_Str(PyObject *);
 #ifdef Py_USING_UNICODE
 PyAPI_FUNC(PyObject *) PyObject_Unicode(PyObject *);
 #endif
+PyAPI_DATA(Py_ssize_t) PyObject_GetBState(PyObject *);
 PyAPI_FUNC(int) PyObject_Compare(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_RichCompare(PyObject *, PyObject *, int);
 PyAPI_FUNC(int) PyObject_RichCompareBool(PyObject *, PyObject *, int);
diff --git a/Lib/test/test_py3kwarn.py b/Lib/test/test_py3kwarn.py
index 9ec40a0b64b697..1a14e8ab345a7b 100644
--- a/Lib/test/test_py3kwarn.py
+++ b/Lib/test/test_py3kwarn.py
@@ -217,6 +217,13 @@ def test_str_parsing(self):
         with check_py3k_warnings():
             "{0}-{1}: {2}".decode()
 
+    def test_str_join(self):
+        with check_py3k_warnings():
+            a = "x" + u"y"
+            a = b"x" + u"y"
+            a = u"x" + b"y"
+            a = u"x" + "y"
+
     def test_string_parsing(self):
         with check_py3k_warnings():
             b"{0}-{1}: {2}"._formatter_parser()
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index af93d00daae3c9..7c2a8c77be32cb 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -80,6 +80,33 @@ test_config(PyObject *self)
     return Py_None;
 }
 
+static PyObject*
+test_join_api(PyObject *self)
+{
+    PyObject *s = PyObject_Str(NULL);
+    PyObject *pieces = PyObject_Unicode(NULL);
+
+    PyObject *result = _PyString_Join(s, pieces);
+    if (((PyUnicodeObject *)result)->ob_bstate != BSTATE_UNICODE) {
+            PyErr_SetString(TestError,
+                            "test_join_api:string bstate update failed");
+            Py_DECREF(result);
+            return (PyObject*)NULL;
+    }
+
+    result = PyUnicode_Join(pieces, s);
+    if (((PyStringObject *)result)->ob_bstate != BSTATE_BYTE) {
+            PyErr_SetString(TestError,
+                            "test_join_api:unicode bstate update failed");
+            Py_DECREF(result);
+            return (PyObject*)NULL;
+    }
+
+    Py_DECREF(result);
+    Py_INCREF(Py_None);
+    return Py_None;
+}
+
 static PyObject*
 test_list_api(PyObject *self)
 {
@@ -2690,6 +2717,7 @@ static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             (PyCFunction)test_config,        METH_NOARGS},
+    {"test_join_api",           (PyCFunction)test_join_api,      METH_NOARGS},
 #if defined(Py_USING_UNICODE) && !defined(Py_BUILD_CORE)
     {"test_datetime_capi",  test_datetime_capi,              METH_NOARGS},
 #endif
diff --git a/Objects/object.c b/Objects/object.c
index 65366b0b351b4c..e057f7e262fab1 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -471,6 +471,57 @@ PyObject_Str(PyObject *v)
     return res;
 }
 
+Py_ssize_t
+PyObject_GetBState(PyObject *v)
+{
+    PyUnicodeObject *uni;
+    PyStringObject *str;
+    PyBytesObject *byt;
+    Py_ssize_t bbstate;
+
+    if (v == NULL) {
+        return 0;
+    }
+    else {
+        if (!PyUnicode_CheckExact(v) || !PyBytes_CheckExact(v)) {
+            return -1;
+        }
+        if (PyUnicode_CheckExact(v)) {
+            uni = (PyUnicodeObject *) PyUnicode_FromObject(v);
+            if (uni == NULL)
+                return 0;
+            bbstate = uni->ob_bstate;
+            if (bbstate == NULL)
+                Py_XDECREF(uni);
+                return 0;
+            Py_DECREF(uni);
+            return bbstate;
+        }
+        if (PyString_CheckExact(v)) {
+            str = (PyStringObject *) v;
+            if (str == NULL)
+                return 0;
+            bbstate = str->ob_bstate;
+            if (bbstate == NULL)
+                Py_XDECREF(str);
+                return 0;
+            Py_DECREF(str);
+            return bbstate;
+        }
+        if (PyBytes_CheckExact(v)) {
+            byt = (PyBytesObject *) v;
+            if (byt == NULL)
+                return 0;
+            bbstate = byt->ob_bstate;
+            if (bbstate == NULL)
+                Py_XDECREF(byt);
+                return 0;
+            Py_DECREF(byt);
+            return bbstate;
+        }
+    }
+}
+
 #ifdef Py_USING_UNICODE
 PyObject *
 PyObject_Unicode(PyObject *v)
@@ -550,6 +601,7 @@ PyObject_Unicode(PyObject *v)
         Py_DECREF(res);
         res = str;
     }
+    ((PyUnicodeObject *)res)->ob_bstate = BSTATE_UNICODE;
     return res;
 }
 #endif
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index d4b536565f760c..4a0920cea33ce6 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -1592,6 +1592,9 @@ string_rsplit(PyStringObject *self, PyObject *args)
 }
 
 
+// Example we are testing
+// m =  "x" + b"y"
+
 PyDoc_STRVAR(join__doc__,
 "S.join(iterable) -> string\n\
 \n\
@@ -1692,6 +1695,14 @@ string_join(PyStringObject *self, PyObject *orig)
     }
 
     Py_DECREF(seq);
+    if (PyUnicode_Check(item)) {
+        ((PyUnicodeObject *)item)->ob_bstate = BSTATE_UNICODE;
+        self->ob_bstate = PyObject_GetBState(item);
+        ((PyUnicodeObject *)res)->ob_bstate = BSTATE_UNICODE;
+
+        if (PyErr_WarnPy3k("joining a String and a Unicode is not supported in 3.x", 1) < 0)
+            return NULL;
+    }
     return res;
 }
 
@@ -3728,6 +3739,12 @@ string_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
     if (x == NULL)
         return PyString_FromString("");
+    if (PyString_CheckExact(((PyStringObject *)x))) {
+        ((PyStringObject *)x)->ob_bstate = BSTATE_NOT_SURE; 
+    } 
+    if (PyBytes_CheckExact(((PyBytesObject *)x))) {
+        ((PyBytesObject *)x)->ob_bstate = BSTATE_BYTE; 
+    }
     return PyObject_Str(x);
 }
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index c00f7f365555d3..cf3afd535ff737 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -374,6 +374,7 @@ PyUnicodeObject *_PyUnicode_New(Py_ssize_t length)
     unicode->length = length;
     unicode->hash = -1;
     unicode->defenc = NULL;
+    unicode->ob_bstate = BSTATE_UNICODE;
     return unicode;
 
   onError:
@@ -5739,6 +5740,13 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
                          i, Py_TYPE(item)->tp_name);
             goto onError;
         }
+        if (PyBytes_CheckExact(item)) {
+            ((PyBytesObject *)item)->ob_bstate = BSTATE_BYTE;
+            ((PyBytesObject *)sep)->ob_bstate = PyObject_GetBState(item);
+
+            if (PyErr_WarnPy3k("joining Unicode and a Byte is not supported in 3.x", 1) < 0)
+                goto onError;
+        }
         item = PyUnicode_FromObject(item);
         if (item == NULL)
             goto onError;
@@ -5791,6 +5799,7 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
   Done:
     Py_XDECREF(internal_separator);
     Py_DECREF(fseq);
+    res->ob_bstate = BSTATE_BYTE;
     return (PyObject *)res;
 
   Overflow:
@@ -8859,6 +8868,7 @@ unicode_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
     if (x == NULL)
         return (PyObject *)_PyUnicode_New(0);
+    ((PyUnicodeObject *)x)->ob_bstate = BSTATE_UNICODE;
     if (encoding == NULL && errors == NULL)
         return PyObject_Unicode(x);
     else