diff -r c14a2d2a3b19 Doc/whatsnew/3.6.rst
--- a/Doc/whatsnew/3.6.rst	Tue Oct 25 10:37:55 2016 +0300
+++ b/Doc/whatsnew/3.6.rst	Tue Oct 25 11:20:30 2016 +0300
@@ -1230,7 +1230,10 @@ Deprecated Python modules, functions and
 Deprecated functions and types of the C API
 -------------------------------------------
 
-* None yet.
+* Undocumented functions :c:func:`PyUnicode_AsEncodedObject`,
+  :c:func:`PyUnicode_AsDecodedObject`, :c:func:`PyUnicode_AsEncodedUnicode`
+  and :c:func:`PyUnicode_AsDecodedUnicode` are deprecated now.
+  Use :ref:`generic codec based API <codec-registry>` instead.
 
 
 Deprecated features
diff -r c14a2d2a3b19 Misc/NEWS
--- a/Misc/NEWS	Tue Oct 25 10:37:55 2016 +0300
+++ b/Misc/NEWS	Tue Oct 25 11:20:30 2016 +0300
@@ -68,6 +68,13 @@ Library
   threadpool executor.
   Initial patch by Hans Lawrenz.
 
+C API
+-----
+
+- Issue #28426: Undocumented functions PyUnicode_AsEncodedObject(),
+  PyUnicode_AsDecodedObject(), PyUnicode_AsDecodedUnicode() and
+  PyUnicode_AsEncodedUnicode() are deprecated now.
+
 Build
 -----
 
diff -r c14a2d2a3b19 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Tue Oct 25 10:37:55 2016 +0300
+++ b/Objects/unicodeobject.c	Tue Oct 25 11:20:30 2016 +0300
@@ -3241,6 +3241,11 @@ PyUnicode_AsDecodedObject(PyObject *unic
         return NULL;
     }
 
+    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                     "PyUnicode_AsDecodedObject() is deprecated; "
+                     "use PyCodec_Decode() to decode from str", 1) < 0)
+        return NULL;
+
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();
 
@@ -3260,6 +3265,11 @@ PyUnicode_AsDecodedUnicode(PyObject *uni
         goto onError;
     }
 
+    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                     "PyUnicode_AsDecodedUnicode() is deprecated; "
+                     "use PyCodec_Decode() to decode from str to str", 1) < 0)
+        return NULL;
+
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();
 
@@ -3310,6 +3320,12 @@ PyUnicode_AsEncodedObject(PyObject *unic
         goto onError;
     }
 
+    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                     "PyUnicode_AsEncodedObject() is deprecated; "
+                     "use PyUnicode_AsEncodedString() to encode from str to bytes "
+                     "or PyCodec_Encode() for generic encoding", 1) < 0)
+        return NULL;
+
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();
 
@@ -3635,6 +3651,11 @@ PyUnicode_AsEncodedUnicode(PyObject *uni
         goto onError;
     }
 
+    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                     "PyUnicode_AsEncodedUnicode() is deprecated; "
+                     "use PyCodec_Encode() to encode from str to str", 1) < 0)
+        return NULL;
+
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();