diff -r 84899daa4309 Include/sysmodule.h
--- a/Include/sysmodule.h	Tue Aug 14 18:41:40 2012 +0300
+++ b/Include/sysmodule.h	Tue Aug 14 23:50:11 2012 +0300
@@ -22,6 +22,7 @@
 
 #ifndef Py_LIMITED_API
 PyAPI_DATA(PyObject *) _PySys_TraceFunc, *_PySys_ProfileFunc;
+PyAPI_DATA(PyObject *) _PySys_GetSizeOf(PyObject *);
 #endif
 
 PyAPI_FUNC(void) PySys_ResetWarnOptions(void);
diff -r 84899daa4309 Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Tue Aug 14 18:41:40 2012 +0300
+++ b/Lib/test/test_memoryio.py	Tue Aug 14 23:50:11 2012 +0300
@@ -9,6 +9,8 @@
 import io
 import _pyio as pyio
 import pickle
+import sys
+import os
 
 class MemorySeekTestMixin:
 
@@ -712,6 +714,25 @@
         memio.close()
         self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
 
+    check_sizeof = support.check_sizeof
+
+    @support.cpython_only
+    def test_sizeof(self):
+        basesize = support.calcobjsize('P2PP4c5P')
+        usize = len('\0'.encode('unicode-internal'))
+        stdsize = basesize + 2 * usize + sys.getsizeof(os.linesep)
+        check = self.check_sizeof
+        self.assertEqual(object.__sizeof__(io.StringIO()), basesize)
+        check(io.StringIO(), stdsize)
+        # newline
+        for newline in '', '\n', '\r', '\r\n':
+            check(io.StringIO(newline=newline),
+                basesize + 2 * usize + sys.getsizeof(newline))
+        # UCS4 buffer
+        for c in 'a', '\x80', '\u0100', '\U00010000':
+            check(io.StringIO(c * 1000),
+                  stdsize + len((c * 1000).encode('unicode-internal')))
+
 
 class CStringIOPickleTest(PyStringIOPickleTest):
     UnsupportedOperation = io.UnsupportedOperation
diff -r 84899daa4309 Modules/_io/stringio.c
--- a/Modules/_io/stringio.c	Tue Aug 14 18:41:40 2012 +0300
+++ b/Modules/_io/stringio.c	Tue Aug 14 23:50:11 2012 +0300
@@ -500,6 +500,7 @@
 static int
 stringio_clear(stringio *self)
 {
+    Py_CLEAR(self->decoder);
     Py_CLEAR(self->dict);
     return 0;
 }
@@ -649,6 +650,35 @@
     return 0;
 }
 
+#define ADD_NUM(res,num) do {                   \
+        PyObject *x = res;                      \
+        PyObject *y = (num);                    \
+        if (x == NULL)                          \
+            return NULL;                        \
+        res = PyNumber_InPlaceAdd(x, y);        \
+        Py_DECREF(x);                           \
+        if (res == NULL) {                      \
+            Py_DECREF(y);                       \
+            return NULL;                        \
+        }                                       \
+    } while (0)
+
+static PyObject *
+stringio_sizeof(stringio *self, void *unused)
+{
+    PyObject *res;
+
+    res = PyLong_FromSsize_t(sizeof(stringio));
+    if (res == NULL)
+        return NULL;
+
+    if (self->buf)
+        ADD_NUM(res, PyLong_FromSsize_t(self->buf_size * sizeof(Py_UNICODE)));
+    if (self->readnl)
+        ADD_NUM(res, _PySys_GetSizeOf(self->readnl));
+    return res;
+}
+
 /* Properties and pseudo-properties */
 static PyObject *
 stringio_seekable(stringio *self, PyObject *args)
@@ -841,6 +871,7 @@
 
     {"__getstate__", (PyCFunction)stringio_getstate, METH_NOARGS},
     {"__setstate__", (PyCFunction)stringio_setstate, METH_O},
+    {"__sizeof__", (PyCFunction)stringio_sizeof, METH_NOARGS},
     {NULL, NULL}        /* sentinel */
 };
 
diff -r 84899daa4309 Python/sysmodule.c
--- a/Python/sysmodule.c	Tue Aug 14 18:41:40 2012 +0300
+++ b/Python/sysmodule.c	Tue Aug 14 23:50:11 2012 +0300
@@ -807,19 +807,13 @@
 }
 #endif /* USE_MALLOPT */
 
-static PyObject *
-sys_getsizeof(PyObject *self, PyObject *args, PyObject *kwds)
+PyObject *
+_PySys_GetSizeOf(PyObject *o)
 {
     PyObject *res = NULL;
     static PyObject *str__sizeof__ = NULL, *gc_head_size = NULL;
-    static char *kwlist[] = {"object", "default", 0};
-    PyObject *o, *dflt = NULL;
     PyObject *method;
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O:getsizeof",
-                                     kwlist, &o, &dflt))
-        return NULL;
-
     /* Initialize static variable for GC head size */
     if (gc_head_size == NULL) {
         gc_head_size = PyLong_FromSsize_t(sizeof(PyGC_Head));
@@ -844,6 +838,31 @@
         Py_DECREF(method);
     }
 
+    if (res == NULL)
+        return res;
+
+    /* add gc_head size */
+    if (PyObject_IS_GC(o)) {
+        PyObject *tmp = res;
+        res = PyNumber_Add(tmp, gc_head_size);
+        Py_DECREF(tmp);
+    }
+    return res;
+}
+
+static PyObject *
+sys_getsizeof(PyObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"object", "default", 0};
+    PyObject *res;
+    PyObject *o, *dflt = NULL;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|O:getsizeof",
+                                     kwlist, &o, &dflt))
+        return NULL;
+
+    res = _PySys_GetSizeOf(o);
+
     /* Has a default value been given */
     if ((res == NULL) && (dflt != NULL) &&
         PyErr_ExceptionMatches(PyExc_TypeError))
@@ -852,15 +871,7 @@
         Py_INCREF(dflt);
         return dflt;
     }
-    else if (res == NULL)
-        return res;
 
-    /* add gc_head size */
-    if (PyObject_IS_GC(o)) {
-        PyObject *tmp = res;
-        res = PyNumber_Add(tmp, gc_head_size);
-        Py_DECREF(tmp);
-    }
     return res;
 }