diff -r 0a329f94448d Lib/sysconfig.py
--- a/Lib/sysconfig.py	Mon Nov 11 15:08:40 2013 +0000
+++ b/Lib/sysconfig.py	Mon Nov 11 14:55:01 2013 -0500
@@ -409,6 +409,10 @@
     # _sysconfigdata is generated at build time, see _generate_posix_vars()
     from _sysconfigdata import build_time_vars
     vars.update(build_time_vars)
+    # For backward compatibility, see issue19555
+    SO = build_time_vars.get('EXT_SUFFIX')
+    if SO is not None:
+        vars['SO'] = SO
 
 def _init_non_posix(vars):
     """Initialize the module as appropriate for NT"""
@@ -579,6 +583,9 @@
 
     Equivalent to get_config_vars().get(name)
     """
+    if name == 'SO':
+        import warnings
+        warnings.warn('SO is deprecated, use EXT_SUFFIX', DeprecationWarning)
     return get_config_vars().get(name)
 
 
diff -r 0a329f94448d Lib/test/test_sysconfig.py
--- a/Lib/test/test_sysconfig.py	Mon Nov 11 15:08:40 2013 +0000
+++ b/Lib/test/test_sysconfig.py	Mon Nov 11 14:55:01 2013 -0500
@@ -369,6 +369,25 @@
             os.chdir(cwd)
         self.assertEqual(srcdir, srcdir2)
 
+    @unittest.skipIf(sysconfig.get_config_var('EXT_SUFFIX') is None,
+                     'EXT_SUFFIX required for this test')
+    def test_SO_deprecation(self):
+        self.assertWarns(DeprecationWarning,
+                         sysconfig.get_config_var, 'SO')
+
+    @unittest.skipIf(sysconfig.get_config_var('EXT_SUFFIX') is None,
+                     'EXT_SUFFIX required for this test')
+    def test_SO_value(self):
+        self.assertEqual(sysconfig.get_config_var('SO'),
+                         sysconfig.get_config_var('EXT_SUFFIX'))
+
+    @unittest.skipIf(sysconfig.get_config_var('EXT_SUFFIX') is None,
+                     'EXT_SUFFIX required for this test')
+    def test_SO_in_vars(self):
+        vars = sysconfig.get_config_vars()
+        self.assertIsNotNone(vars['SO'])
+        self.assertEqual(vars['SO'], vars['EXT_SUFFIX'])
+
 
 class MakefileTests(unittest.TestCase):