diff --git a/Lib/test/test_fileio.py b/Lib/test/test_fileio.py
index cbc71657070..c9787795e33 100644
--- a/Lib/test/test_fileio.py
+++ b/Lib/test/test_fileio.py
@@ -50,7 +50,7 @@ class AutoFileTests(unittest.TestCase):
         # verify expected attributes exist
         f = self.f
 
-        self.assertEquals(f.mode, "w")
+        self.assertEquals(f.mode, "wb")
         self.assertEquals(f.closed, False)
 
         # verify the attributes are readonly
@@ -160,7 +160,7 @@ class OtherFileTests(unittest.TestCase):
 
     def testModeStrings(self):
         # check invalid mode strings
-        for mode in ("", "aU", "wU+", "rb", "rt"):
+        for mode in ("", "aU", "wU+", "rw", "rt"):
             try:
                 f = _fileio._FileIO(TESTFN, mode)
             except ValueError:
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index c9bd38ddffb..eb41d1ffa7b 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1266,7 +1266,7 @@ class MiscIOTest(unittest.TestCase):
 
     def test_attributes(self):
         f = io.open(test_support.TESTFN, "wb", buffering=0)
-        self.assertEquals(f.mode, "w")
+        self.assertEquals(f.mode, "wb")
         f.close()
 
         f = io.open(test_support.TESTFN, "U")
@@ -1274,18 +1274,18 @@ class MiscIOTest(unittest.TestCase):
         self.assertEquals(f.buffer.name,     test_support.TESTFN)
         self.assertEquals(f.buffer.raw.name, test_support.TESTFN)
         self.assertEquals(f.mode,            "U")
-        self.assertEquals(f.buffer.mode,     "r")
-        self.assertEquals(f.buffer.raw.mode, "r")
+        self.assertEquals(f.buffer.mode,     "rb")
+        self.assertEquals(f.buffer.raw.mode, "rb")
         f.close()
 
         f = io.open(test_support.TESTFN, "w+")
         self.assertEquals(f.mode,            "w+")
-        self.assertEquals(f.buffer.mode,     "r+") # Does it really matter?
-        self.assertEquals(f.buffer.raw.mode, "r+")
+        self.assertEquals(f.buffer.mode,     "rb+") # Does it really matter?
+        self.assertEquals(f.buffer.raw.mode, "rb+")
 
         g = io.open(f.fileno(), "wb", closefd=False)
-        self.assertEquals(g.mode,     "w")
-        self.assertEquals(g.raw.mode, "w")
+        self.assertEquals(g.mode,     "wb")
+        self.assertEquals(g.raw.mode, "wb")
         self.assertEquals(g.name,     f.fileno())
         self.assertEquals(g.raw.name, f.fileno())
         f.close()
diff --git a/Misc/NEWS b/Misc/NEWS
index d691d8cce22..c3f0ec61e1b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -56,6 +56,8 @@ Library
 - Issue #4363: The uuid.uuid1() and uuid.uuid4() functions now work even if
   the ctypes module is not present.
 
+- FileIO's mode attribute now always includes ``"b"``.
+
 - Issue #4116: Resolve member name conflict in ScrolledCanvas.__init__.
 
 - httplib.HTTPConnection.putheader() now accepts an arbitrary number of values
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index b9310f380ca..65cc99d45a8 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -208,6 +208,8 @@ fileio_init(PyObject *oself, PyObject *args, PyObject *kwds)
 			flags |= O_CREAT;
 			append = 1;
 			break;
+		case 'b':
+			break;
 		case '+':
 			if (plus)
 				goto bad_mode;
@@ -682,12 +684,12 @@ mode_string(PyFileIOObject *self)
 {
 	if (self->readable) {
 		if (self->writable)
-			return "r+";
+			return "rb+";
 		else
-			return "r";
+			return "rb";
 	}
 	else
-		return "w";
+		return "wb";
 }
 
 static PyObject *