diff -r 2159ff4595b7 Lib/json/encoder.py
--- a/Lib/json/encoder.py	Fri Oct 26 19:35:00 2012 +0300
+++ b/Lib/json/encoder.py	Fri Oct 26 17:14:21 2012 -0400
@@ -274,7 +274,7 @@
         if _indent is not None:
             _current_indent_level += 1
             newline_indent = '\n' + _indent * _current_indent_level
-            separator = _item_separator + newline_indent
+            separator = _item_separator.rstrip() + newline_indent
             buf += newline_indent
         else:
             newline_indent = None
@@ -326,7 +326,7 @@
         if _indent is not None:
             _current_indent_level += 1
             newline_indent = '\n' + _indent * _current_indent_level
-            item_separator = _item_separator + newline_indent
+            item_separator = _item_separator.rstrip() + newline_indent
             yield newline_indent
         else:
             newline_indent = None
diff -r 2159ff4595b7 Lib/test/json_tests/test_indent.py
--- a/Lib/test/json_tests/test_indent.py	Fri Oct 26 19:35:00 2012 +0300
+++ b/Lib/test/json_tests/test_indent.py	Fri Oct 26 17:14:21 2012 -0400
@@ -58,6 +58,28 @@
         # indent=None is more compact
         check(None, '{"3": 1}')
 
+    def test_no_trailing_space_after_list_item(self):
+        # Using default separator
+        output = self.dumps(['foo', 'bar'], indent=2)
+        expected = '[\n  "foo",\n  "bar"\n]'
+        self.assertEqual(output, expected)
+
+        # Using custom separator
+        output = self.dumps(['foo', 'bar'], indent=2, separators=('   ,   ', ': '))
+        expected = '[\n  "foo"   ,\n  "bar"\n]'
+        self.assertEqual(output, expected)
+
+    def test_no_trailing_space_after_dict_item(self):
+        # Using default separator
+        output = self.dumps({'foo': 1, 'bar': 2}, indent=2)
+        expected = '{\n  "foo": 1,\n  "bar": 2\n}'
+        self.assertEqual(output, expected)
+
+        # Using custom separator
+        output = self.dumps({'foo': 1, 'bar': 2}, indent=2, separators=('   ,   ', ': '))
+        expected = '{\n  "foo": 1   ,\n  "bar": 2\n}'
+        self.assertEqual(output, expected)
+
 
 class TestPyIndent(TestIndent, PyTest): pass
 class TestCIndent(TestIndent, CTest): pass