diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1194,7 +1194,7 @@ class EscapeDecodeTest(unittest.TestCase):
         check(b"[\\\n]", b"[]")
         check(br'[\"]', b'["]')
         check(br"[\']", b"[']")
-        check(br"[\\]", br"[\]")
+        check(br"[\\]", b"[\\]")
         check(br"[\a]", b"[\x07]")
         check(br"[\b]", b"[\x08]")
         check(br"[\t]", b"[\x09]")
@@ -1203,7 +1203,6 @@ class EscapeDecodeTest(unittest.TestCase):
         check(br"[\f]", b"[\x0c]")
         check(br"[\r]", b"[\x0d]")
         check(br"[\7]", b"[\x07]")
-        check(br"[\8]", br"[\8]")
         check(br"[\78]", b"[\x078]")
         check(br"[\41]", b"[!]")
         check(br"[\418]", b"[!8]")
@@ -1211,12 +1210,18 @@ class EscapeDecodeTest(unittest.TestCase):
         check(br"[\1010]", b"[A0]")
         check(br"[\501]", b"[A]")
         check(br"[\x41]", b"[A]")
-        check(br"[\X41]", br"[\X41]")
         check(br"[\x410]", b"[A0]")
-        for b in range(256):
-            if b not in b'\n"\'\\abtnvfr01234567x':
-                b = bytes([b])
-                check(b'\\' + b, b'\\' + b)
+        for i in range(97, 123):
+            b = bytes([i])
+            if b not in b'abfnrtvx':
+                with self.assertWarns(DeprecationWarning):
+                    check(b"\\" + b, b"\\" + b)
+            with self.assertWarns(DeprecationWarning):
+                check(b"\\" + b.upper(), b"\\" + b.upper())
+        with self.assertWarns(DeprecationWarning):
+            check(br"\8", b"\\8")
+        with self.assertWarns(DeprecationWarning):
+            check(br"\9", b"\\9")
 
     def test_errors(self):
         decode = codecs.escape_decode
@@ -2465,7 +2470,6 @@ class UnicodeEscapeTest(unittest.TestCase):
         check(br"[\f]", "[\x0c]")
         check(br"[\r]", "[\x0d]")
         check(br"[\7]", "[\x07]")
-        check(br"[\8]", r"[\8]")
         check(br"[\78]", "[\x078]")
         check(br"[\41]", "[!]")
         check(br"[\418]", "[!8]")
@@ -2475,9 +2479,18 @@ class UnicodeEscapeTest(unittest.TestCase):
         check(br"[\x410]", "[A0]")
         check(br"\u20ac", "\u20ac")
         check(br"\U0001d120", "\U0001d120")
-        for b in range(256):
-            if b not in b'\n"\'\\abtnvfr01234567xuUN':
-                check(b'\\' + bytes([b]), '\\' + chr(b))
+        for i in range(97, 123):
+            b = bytes([i])
+            if b not in b'abfnrtuvx':
+                with self.assertWarns(DeprecationWarning):
+                    check(b"\\" + b, "\\" + chr(i))
+            if b.upper() not in b'UN':
+                with self.assertWarns(DeprecationWarning):
+                    check(b"\\" + b.upper(), "\\" + chr(i-32))
+        with self.assertWarns(DeprecationWarning):
+            check(br"\8", "\\8")
+        with self.assertWarns(DeprecationWarning):
+            check(br"\9", "\\9")
 
     def test_decode_errors(self):
         decode = codecs.unicode_escape_decode
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
@@ -10,6 +10,7 @@ import codecs
 import itertools
 import operator
 import struct
+import string
 import sys
 import unittest
 import warnings
@@ -2729,6 +2730,12 @@ class UnicodeTest(string_tests.CommonTest,
         support.check_free_after_iterating(self, iter, str)
         support.check_free_after_iterating(self, reversed, str)
 
+    def test_invalid_sequences(self):
+        for letter in string.ascii_letters + "89": # 0-7 are octal escapes
+            if letter in "abfnrtuvxNU":
+                continue
+            with self.assertWarns(DeprecationWarning):
+                eval(r"'\%s'" % letter)
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -1207,8 +1207,9 @@ PyObject *PyBytes_DecodeEscape(const char *s,
             break;
 
         default:
+            if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1, "invalid escape sequence '%c'", *(--s)) < 0)
+                goto failed;
             *p++ = '\\';
-            s--;
             goto non_esc; /* an arbitrary number of unescaped
                              UTF-8 bytes may follow. */
         }
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5849,6 +5849,7 @@ PyUnicode_DecodeUnicodeEscape(const char *s,
     _PyUnicodeWriter writer;
     const char *end;
     char* message;
+    int ret;
     Py_UCS4 chr = 0xffffffff; /* in case 'getcode' messes up */
     PyObject *errorHandler = NULL;
     PyObject *exc = NULL;
@@ -6023,6 +6024,11 @@ PyUnicode_DecodeUnicodeEscape(const char *s,
                 goto error;
             }
             else {
+                /* Deprecate invalid escape sequences */
+                ret = PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+                                       "invalid escape sequence '%c'", c);
+                if (ret < 0)
+                    goto onError;
                 WRITECHAR('\\');
                 WRITECHAR((unsigned char)s[-1]);
             }