diff --git a/scrapy/utils/reqser.py b/scrapy/utils/reqser.py
index 959dddbd5..3c463cfed 100644
--- a/scrapy/utils/reqser.py
+++ b/scrapy/utils/reqser.py
@@ -67,6 +67,20 @@ def request_from_dict(d, spider=None):
         flags=d.get('flags'))
 
 
+def _is_private_method(name):
+    return name.startswith('__') and not name.endswith('__')
+
+
+def _mangle_private_name(obj, func, name):
+    qualname = getattr(func, '__qualname__', None)
+    if qualname is None:
+        classname = obj.__class__.__name__.lstrip('_')
+        return '_%s%s' % (classname, name)
+    else:
+        splits = qualname.split('.')
+        return '_%s%s' % (splits[-2], splits[-1])
+
+
 def _find_method(obj, func):
     if obj:
         try:
@@ -75,7 +89,10 @@ def _find_method(obj, func):
             pass
         else:
             if func_self is obj:
-                return six.get_method_function(func).__name__
+                name = six.get_method_function(func).__name__
+                if _is_private_method(name):
+                    return _mangle_private_name(obj, func, name)
+                return name
     raise ValueError("Function %s is not a method of: %s" % (func, obj))
 
 
diff --git a/tests/test_utils_reqser.py b/tests/test_utils_reqser.py
index dcc070b8f..e5a09dcf1 100644
--- a/tests/test_utils_reqser.py
+++ b/tests/test_utils_reqser.py
@@ -1,9 +1,12 @@
 # -*- coding: utf-8 -*-
 import unittest
+import sys
+
+import six
 
 from scrapy.http import Request, FormRequest
 from scrapy.spiders import Spider
-from scrapy.utils.reqser import request_to_dict, request_from_dict
+from scrapy.utils.reqser import request_to_dict, request_from_dict, _is_private_method, _mangle_private_name
 
 
 class RequestSerializationTest(unittest.TestCase):
@@ -68,6 +71,56 @@ class RequestSerializationTest(unittest.TestCase):
                     errback=self.spider.handle_error)
         self._assert_serializes_ok(r, spider=self.spider)
 
+    def test_private_callback_serialization(self):
+        r = Request("http://www.example.com",
+                    callback=self.spider._TestSpider__parse_item_private,
+                    errback=self.spider.handle_error)
+        self._assert_serializes_ok(r, spider=self.spider)
+
+    def test_mixin_private_callback_serialization(self):
+        if sys.version_info[0] < 3:
+            return
+        r = Request("http://www.example.com",
+                    callback=self.spider._TestSpiderMixin__mixin_callback,
+                    errback=self.spider.handle_error)
+        self._assert_serializes_ok(r, spider=self.spider)
+
+    def test_private_callback_name_matching(self):
+        self.assertTrue(_is_private_method('__a'))
+        self.assertTrue(_is_private_method('__a_'))
+        self.assertTrue(_is_private_method('__a_a'))
+        self.assertTrue(_is_private_method('__a_a_'))
+        self.assertTrue(_is_private_method('__a__a'))
+        self.assertTrue(_is_private_method('__a__a_'))
+        self.assertTrue(_is_private_method('__a___a'))
+        self.assertTrue(_is_private_method('__a___a_'))
+        self.assertTrue(_is_private_method('___a'))
+        self.assertTrue(_is_private_method('___a_'))
+        self.assertTrue(_is_private_method('___a_a'))
+        self.assertTrue(_is_private_method('___a_a_'))
+        self.assertTrue(_is_private_method('____a_a_'))
+
+        self.assertFalse(_is_private_method('_a'))
+        self.assertFalse(_is_private_method('_a_'))
+        self.assertFalse(_is_private_method('__a__'))
+        self.assertFalse(_is_private_method('__'))
+        self.assertFalse(_is_private_method('___'))
+        self.assertFalse(_is_private_method('____'))
+
+    def _assert_mangles_to(self, obj, name):
+        func = getattr(obj, name)
+        self.assertEqual(
+            _mangle_private_name(obj, func, func.__name__),
+            name
+        )
+
+    def test_private_name_mangling(self):
+        self._assert_mangles_to(
+            self.spider, '_TestSpider__parse_item_private')
+        if sys.version_info[0] >= 3:
+            self._assert_mangles_to(
+                self.spider, '_TestSpiderMixin__mixin_callback')
+
     def test_unserializable_callback1(self):
         r = Request("http://www.example.com", callback=lambda x: x)
         self.assertRaises(ValueError, request_to_dict, r)
@@ -78,7 +131,12 @@ class RequestSerializationTest(unittest.TestCase):
         self.assertRaises(ValueError, request_to_dict, r)
 
 
-class TestSpider(Spider):
+class TestSpiderMixin(object):
+    def __mixin_callback(self, response):
+        pass
+
+
+class TestSpider(Spider, TestSpiderMixin):
     name = 'test'
 
     def parse_item(self, response):
@@ -87,6 +145,9 @@ class TestSpider(Spider):
     def handle_error(self, failure):
         pass
 
+    def __parse_item_private(self, response):
+        pass
+
 
 class CustomRequest(Request):
     pass