diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 5458f99..04aed8d 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -765,6 +765,11 @@
         self._buffer = buf
         self._pos = 0
 
+    def __getstate__(self):
+        if self.closed:
+            raise ValueError("__getstate__ on closed file")
+        return self.__dict__.copy()
+
     def getvalue(self):
         """Return the bytes value (contents) of the buffer
         """
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 670dab9..7dd1658 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -9,6 +9,7 @@
 import io
 import _pyio as pyio
 import sys
+import pickle
 
 class MemorySeekTestMixin:
 
@@ -346,6 +347,42 @@
             memio = self.ioclass()
             memio.foo = 1
 
+    def test_pickling(self):
+        buf = self.buftype("1234567890")
+        memio = self.ioclass(buf)
+        memio.foo = 42
+        memio.seek(2)
+
+        class PickleTestMemIO(self.ioclass):
+            def __init__(me, initvalue, foo):
+                self.ioclass.__init__(me, initvalue)
+                me.foo = foo
+            # __getnewargs__ is undefined on purpose. This checks that PEP 307
+            # is used to provide pickling support.
+
+        # Pickle expects the class to be on the module level. Here we use a
+        # little hack to allow the PickleTestMemIO class to derive from
+        # self.ioclass without having to define all combinations explictly on
+        # the module-level.
+        import __main__
+        PickleTestMemIO.__module__ = '__main__'
+        __main__.PickleTestMemIO = PickleTestMemIO
+        submemio = PickleTestMemIO(buf, 80)
+        submemio.seek(2)
+
+        # We only support pickle protocol 2 and onward since we use extended
+        # __reduce__ API of PEP 307 to provide pickling support.
+        for proto in range(2, pickle.HIGHEST_PROTOCOL):
+            for obj in (memio, submemio):
+                obj2 = pickle.loads(pickle.dumps(obj, protocol=proto))
+                self.assertEqual(obj.getvalue(), obj2.getvalue())
+                self.assertEqual(obj.__class__, obj2.__class__)
+                self.assertEqual(obj.foo, obj2.foo)
+                self.assertEqual(obj.tell(), obj2.tell())
+                obj.close()
+                self.assertRaises(ValueError, pickle.dumps, obj, proto)
+        del __main__.PickleTestMemIO
+
 
 class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
 
@@ -425,13 +462,26 @@
         self.assertEqual(memio.getvalue(), buf)
 
 
-class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
-    buftype = str
-    ioclass = pyio.StringIO
-    UnsupportedOperation = pyio.UnsupportedOperation
-    EOF = ""
+class TextIOTestMixin:
 
-    # TextIO-specific behaviour.
+    def test_relative_seek(self):
+        memio = self.ioclass()
+
+        self.assertRaises(IOError, memio.seek, -1, 1)
+        self.assertRaises(IOError, memio.seek, 3, 1)
+        self.assertRaises(IOError, memio.seek, -3, 1)
+        self.assertRaises(IOError, memio.seek, -1, 2)
+        self.assertRaises(IOError, memio.seek, 1, 1)
+        self.assertRaises(IOError, memio.seek, 1, 2)
+
+    def test_textio_properties(self):
+        memio = self.ioclass()
+
+        # These are just dummy values but we nevertheless check them for fear
+        # of unexpected breakage.
+        self.assertTrue(memio.encoding is None)
+        self.assertEqual(memio.errors, "strict")
+        self.assertEqual(memio.line_buffering, False)
 
     def test_newlines_property(self):
         memio = self.ioclass(newline=None)
@@ -513,7 +563,6 @@
     def test_newline_cr(self):
         # newline="\r"
         memio = self.ioclass("a\nb\r\nc\rd", newline="\r")
-        memio.seek(0)
         self.assertEqual(memio.read(), "a\rb\r\rc\rd")
         memio.seek(0)
         self.assertEqual(list(memio), ["a\r", "b\r", "\r", "c\r", "d"])
@@ -521,7 +570,6 @@
     def test_newline_crlf(self):
         # newline="\r\n"
         memio = self.ioclass("a\nb\r\nc\rd", newline="\r\n")
-        memio.seek(0)
         self.assertEqual(memio.read(), "a\r\nb\r\r\nc\rd")
         memio.seek(0)
         self.assertEqual(list(memio), ["a\r\n", "b\r\r\n", "c\rd"])
@@ -539,10 +587,59 @@
             self.ioclass(newline=newline)
 
 
+class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin,
+                     TextIOTestMixin, unittest.TestCase):
+    buftype = str
+    ioclass = pyio.StringIO
+    UnsupportedOperation = pyio.UnsupportedOperation
+    EOF = ""
+
+
+class PyStringIOPickleTest(TextIOTestMixin, unittest.TestCase):
+    """Test if pickle restores properly the internal state of StringIO.
+    """
+    buftype = str
+    UnsupportedOperation = pyio.UnsupportedOperation
+    EOF = ""
+
+    class ioclass(pyio.StringIO):
+        def __new__(cls, *args, **kwargs):
+            return pickle.loads(pickle.dumps(pyio.StringIO(*args, **kwargs)))
+        def __init__(self, *args, **kwargs):
+            pass
+
+
 class CBytesIOTest(PyBytesIOTest):
     ioclass = io.BytesIO
     UnsupportedOperation = io.UnsupportedOperation
 
+    def test_getstate(self):
+        memio = self.ioclass()
+        state = memio.__getstate__()
+        self.assertEqual(len(state), 3)
+        bytearray(state[0]) # Check if state[0] supports the buffer interface.
+        self.assert_(isinstance(state[1], int))
+        self.assert_(isinstance(state[2], dict) or state[2] is None)
+        memio.close()
+        self.assertRaises(ValueError, memio.__getstate__)
+
+    def test_setstate(self):
+        # This checks whether __setstate__ does proper input validation.
+        memio = self.ioclass()
+        memio.__setstate__((b"no error", 0, None))
+        memio.__setstate__((bytearray(b"no error"), 0, None))
+        memio.__setstate__((b"no error", 0, {'spam': 3}))
+        self.assertRaises(ValueError, memio.__setstate__, (b"", -1, None))
+        self.assertRaises(TypeError, memio.__setstate__, ("unicode", 0, None))
+        self.assertRaises(TypeError, memio.__setstate__, (b"", 0.0, None))
+        self.assertRaises(TypeError, memio.__setstate__, (b"", 0, 0))
+        self.assertRaises(TypeError, memio.__setstate__, (b"len-test", 0))
+        self.assertRaises(TypeError, memio.__setstate__)
+        self.assertRaises(TypeError, memio.__setstate__, 0)
+        memio.close()
+        self.assertRaises(ValueError, memio.__setstate__, (b"closed", 0, None))
+
+
 class CStringIOTest(PyStringIOTest):
     ioclass = io.StringIO
     UnsupportedOperation = io.UnsupportedOperation
@@ -561,9 +658,48 @@
         self.assertEqual(memio.tell(), len(buf) * 2)
         self.assertEqual(memio.getvalue(), buf + buf)
 
+    def test_getstate(self):
+        memio = self.ioclass()
+        state = memio.__getstate__()
+        self.assertEqual(len(state), 4)
+        self.assert_(isinstance(state[0], str))
+        self.assert_(isinstance(state[1], str))
+        self.assert_(isinstance(state[2], int))
+        self.assert_(isinstance(state[3], dict) or state[3] is None)
+        memio.close()
+        self.assertRaises(ValueError, memio.__getstate__)
+
+    def test_setstate(self):
+        # This checks whether __setstate__ does proper input validation.
+        memio = self.ioclass()
+        memio.__setstate__(("no error", "\n", 0, None))
+        memio.__setstate__(("no error", "", 0, {'spam': 3}))
+        self.assertRaises(ValueError, memio.__setstate__, ("", "f", 0, None))
+        self.assertRaises(ValueError, memio.__setstate__, ("", "", -1, None))
+        self.assertRaises(TypeError, memio.__setstate__, (b"", "", 0, None))
+        self.assertRaises(TypeError, memio.__setstate__, ("", b"", 0, None))
+        self.assertRaises(TypeError, memio.__setstate__, ("", "", 0.0, None))
+        self.assertRaises(TypeError, memio.__setstate__, ("", "", 0, 0))
+        self.assertRaises(TypeError, memio.__setstate__, ("len-test", 0))
+        self.assertRaises(TypeError, memio.__setstate__)
+        self.assertRaises(TypeError, memio.__setstate__, 0)
+        memio.close()
+        self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
+
+
+class CStringIOPickleTest(PyStringIOPickleTest):
+    UnsupportedOperation = io.UnsupportedOperation
+
+    class ioclass(io.StringIO):
+        def __new__(cls, *args, **kwargs):
+            return pickle.loads(pickle.dumps(io.StringIO(*args, **kwargs)))
+        def __init__(self, *args, **kwargs):
+            pass
+
 
 def test_main():
-    tests = [PyBytesIOTest, PyStringIOTest, CBytesIOTest, CStringIOTest]
+    tests = [PyBytesIOTest, PyStringIOTest, CBytesIOTest, CStringIOTest,
+             PyStringIOPickleTest, CStringIOPickleTest]
     support.run_unittest(*tests)
 
 if __name__ == '__main__':
diff --git a/Misc/NEWS b/Misc/NEWS
index 12d18af..01ca4d1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -85,6 +85,8 @@
 - Issue #4005: Fixed a crash of pydoc when there was a zip file present in
   sys.path.
 
+- Issue #6218: io.StringIO and io.BytesIO instances are now picklable.
+
 Extension Modules
 -----------------
 
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index daef78f..7b56551 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -606,6 +606,120 @@
     Py_RETURN_NONE;
 }
 
+/* Pickling support.
+
+   Note that only pickle protocol 2 and onward are supported since we use
+   extended __reduce__ API of PEP 307 to make BytesIO instances picklable.
+
+   Providing support for protocol < 2 would require the __reduce_ex__ method
+   which is notably long-winded when defined properly.
+
+   For BytesIO, the implementation would similar to one coded for
+   object.__reduce_ex__, but slightly less general. To be more specific, we
+   could call bytesio_getstate directly and avoid checking for the presence of
+   a fallback __reduce__ method. However, we would still need a __newobj__
+   function to use the efficient instance representation of PEP 307.
+ */
+
+static PyObject *
+bytesio_getstate(bytesio *self)
+{
+    PyObject *initvalue = bytesio_getvalue(self);
+    PyObject *dict;
+    PyObject *state;
+
+    if (initvalue == NULL)
+        return NULL;
+    if (self->dict == NULL) {
+        Py_INCREF(Py_None);
+        dict = Py_None;
+    }
+    else {
+        dict = PyDict_Copy(self->dict);
+        if (dict == NULL)
+            return NULL;
+    }
+
+    state = Py_BuildValue("(OnN)", initvalue, self->pos, dict);
+    Py_DECREF(initvalue);
+    return state;
+}
+
+static PyObject *
+bytesio_setstate(bytesio *self, PyObject *state)
+{
+    PyObject *result;
+    PyObject *position_obj;
+    PyObject *dict;
+    Py_ssize_t pos;
+
+    assert(state != NULL);
+
+    /* We allow the state tuple to be longer than 3, because we may need
+       someday to extend the object's state without breaking
+       backward-compatibility. */
+    if (!PyTuple_Check(state) || Py_SIZE(state) < 3) {
+        PyErr_Format(PyExc_TypeError,
+                     "%.200s.__setstate__ argument should be 3-tuple, got %.200s",
+                     Py_TYPE(self)->tp_name, Py_TYPE(state)->tp_name);
+        return NULL;
+    }
+    /* Reset the object to its default state. This is only needed to handle
+       the case of repeated calls to __setstate__. */
+    self->string_size = 0;
+    self->pos = 0;
+
+    /* Set the value of the internal buffer. If state[0] does not support the
+       buffer protocol, bytesio_write will raise the appropriate TypeError. */
+    result = bytesio_write(self, PyTuple_GET_ITEM(state, 0));
+    if (result == NULL)
+        return NULL;
+    Py_DECREF(result);
+
+    /* Set carefully the position value. Alternatively, we could use the seek
+       method instead of modifying self->pos directly to better protect the
+       object internal state against errneous (or malicious) inputs. */
+    position_obj = PyTuple_GET_ITEM(state, 1);
+    if (!PyLong_Check(position_obj)) {
+        PyErr_Format(PyExc_TypeError,
+                     "second item of state must be an integer, not %.200s",
+                     Py_TYPE(position_obj)->tp_name);
+        return NULL;
+    }
+    pos = PyLong_AsSsize_t(position_obj);
+    if (pos == -1 && PyErr_Occurred())
+        return NULL;
+    if (pos < 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "position value cannot be negative");
+        return NULL;
+    }
+    self->pos = pos;
+
+    /* Set the dictionary of the instance variables. */
+    dict = PyTuple_GET_ITEM(state, 2);
+    if (dict != Py_None) {
+        if (!PyDict_Check(dict)) {
+            PyErr_Format(PyExc_TypeError,
+                         "third item of state should be a dict, got a %.200s",
+                         Py_TYPE(dict)->tp_name);
+            return NULL;
+        }
+        if (self->dict) {
+            /* Alternatively, we could replace the internal dictionary
+               completely. However, it seems more practical to just update it. */
+            if (PyDict_Update(self->dict, dict) < 0)
+                return NULL;
+        }
+        else {
+            Py_INCREF(dict);
+            self->dict = dict;
+        }
+    }
+
+    Py_RETURN_NONE;
+}
+
 static void
 bytesio_dealloc(bytesio *self)
 {
@@ -630,9 +744,9 @@
     if (self == NULL)
         return NULL;
 
-    self->string_size = 0;
-    self->pos = 0;
-    self->buf_size = 0;
+    /* tp_alloc initializes all the fields to zero. So we don't have to
+       initialize them here. */
+
     self->buf = (char *)PyMem_Malloc(0);
     if (self->buf == NULL) {
         Py_DECREF(self);
@@ -705,6 +819,8 @@
     {"getvalue",   (PyCFunction)bytesio_getvalue,   METH_VARARGS, getval_doc},
     {"seek",       (PyCFunction)bytesio_seek,       METH_VARARGS, seek_doc},
     {"truncate",   (PyCFunction)bytesio_truncate,   METH_VARARGS, truncate_doc},
+    {"__getstate__",  (PyCFunction)bytesio_getstate,  METH_NOARGS, NULL},
+    {"__setstate__",  (PyCFunction)bytesio_setstate,  METH_O, NULL},
     {NULL, NULL}        /* sentinel */
 };
 
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index bfb099c..ad9730d 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -533,9 +533,9 @@
     if (self == NULL)
         return NULL;
 
-    self->string_size = 0;
-    self->pos = 0;
-    self->buf_size = 0;
+    /* tp_alloc initializes all the fields to zero. So we don't have to
+       initialize them here. */
+
     self->buf = (Py_UNICODE *)PyMem_Malloc(0);
     if (self->buf == NULL) {
         Py_DECREF(self);
@@ -600,6 +600,9 @@
     assert((newline != NULL && newline_obj != Py_None) ||
            (newline == NULL && newline_obj == Py_None));
 
+    assert((newline != NULL && newline_obj != Py_None) ||
+           (newline == NULL && newline_obj == Py_None));
+
     if (newline) {
         self->readnl = PyUnicode_FromString(newline);
         if (self->readnl == NULL)
@@ -672,6 +675,135 @@
     Py_RETURN_TRUE;
 }
 
+/* Pickling support.
+
+   The implementation of __getstate__ is similar to the one for BytesIO,
+   except that we also save the newline parameter. For __setstate__ and unlike
+   BytesIO, we call __init__ to restore the object's state. Doing so allows us
+   to avoid decoding the complex newline state while keeping the object
+   representation compact.
+
+   See comment in bytesio.c regarding why only pickle protocols and onward are
+   supported.
+*/
+
+static PyObject *
+stringio_getstate(stringio *self)
+{
+    PyObject *initvalue = stringio_getvalue(self);
+    PyObject *dict;
+    PyObject *state;
+
+    if (initvalue == NULL)
+        return NULL;
+    if (self->dict == NULL) {
+        Py_INCREF(Py_None);
+        dict = Py_None;
+    }
+    else {
+        dict = PyDict_Copy(self->dict);
+        if (dict == NULL)
+            return NULL;
+    }
+
+    state = Py_BuildValue("(OOnN)", initvalue,
+                          self->readnl ? self->readnl : Py_None,
+                          self->pos, dict);
+    Py_DECREF(initvalue);
+    return state;
+}
+
+static PyObject *
+stringio_setstate(stringio *self, PyObject *state)
+{
+    PyObject *initarg;
+    PyObject *position_obj;
+    PyObject *dict;
+    Py_ssize_t pos;
+
+    assert(state != NULL);
+    CHECK_CLOSED(self);
+
+    /* We allow the state tuple to be longer than 4, because we may need
+       someday to extend the object's state without breaking
+       backward-compatibility. */
+    if (!PyTuple_Check(state) || Py_SIZE(state) < 4) {
+        PyErr_Format(PyExc_TypeError,
+                     "%.200s.__setstate__ argument should be 4-tuple, got %.200s",
+                     Py_TYPE(self)->tp_name, Py_TYPE(state)->tp_name);
+        return NULL;
+    }
+
+    /* Initialize the object's state. */
+    initarg = PyTuple_GetSlice(state, 0, 2);
+    if (initarg == NULL)
+        return NULL;
+    if (stringio_init(self, initarg, NULL) < 0) {
+        Py_DECREF(initarg);
+        return NULL;
+    }
+    Py_DECREF(initarg);
+
+    /* Restore the buffer state. Even if __init__ did initialize the buffer,
+       we have to initialize it again since __init__ may translates the
+       newlines in the inital_value string. We clearly do not want that
+       because the string value in the state tuple has already been translated
+       once by __init__. So we do not take any chance and replace object's
+       buffer completely. */
+    {
+        Py_UNICODE *buf = PyUnicode_AS_UNICODE(PyTuple_GET_ITEM(state, 0));
+        Py_ssize_t bufsize = PyUnicode_GET_SIZE(PyTuple_GET_ITEM(state, 0));
+        if (resize_buffer(self, bufsize) < 0)
+            return NULL;
+        memcpy(self->buf, buf, bufsize * sizeof(Py_UNICODE));
+        self->string_size = bufsize;
+    }
+
+    /* Set carefully the position value. Alternatively, we could use the seek
+       method instead of modifying self->pos directly to better protect the
+       object internal state against errneous (or malicious) inputs. */
+    position_obj = PyTuple_GET_ITEM(state, 2);
+    if (!PyLong_Check(position_obj)) {
+        PyErr_Format(PyExc_TypeError,
+                     "third item of state must be an integer, got %.200s",
+                     Py_TYPE(position_obj)->tp_name);
+        return NULL;
+    }
+    pos = PyLong_AsSsize_t(position_obj);
+    if (pos == -1 && PyErr_Occurred())
+        return NULL;
+    if (pos < 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "position value cannot be negative");
+        return NULL;
+    }
+    self->pos = pos;
+
+    /* Set the dictionary of the instance variables. */
+    dict = PyTuple_GET_ITEM(state, 3);
+    if (dict != Py_None) {
+        if (!PyDict_Check(dict)) {
+            PyErr_Format(PyExc_TypeError,
+                         "fourth item of state should be a dict, got a %.200s",
+                         Py_TYPE(dict)->tp_name);
+            return NULL;
+        }
+        if (self->dict) {
+            /* Alternatively, we could replace the internal dictionary
+               completely. However, it seems more practical to just update it. */
+            if (PyDict_Update(self->dict, dict) < 0)
+                return NULL;
+        }
+        else {
+            Py_INCREF(dict);
+            self->dict = dict;
+        }
+    }
+
+    Py_RETURN_NONE;
+}
+
+
 static PyObject *
 stringio_closed(stringio *self, void *context)
 {
@@ -706,10 +838,13 @@
     {"truncate", (PyCFunction)stringio_truncate, METH_VARARGS, stringio_truncate_doc},
     {"seek",     (PyCFunction)stringio_seek,     METH_VARARGS, stringio_seek_doc},
     {"write",    (PyCFunction)stringio_write,    METH_O,       stringio_write_doc},
-    
+
     {"seekable", (PyCFunction)stringio_seekable, METH_NOARGS},
     {"readable", (PyCFunction)stringio_readable, METH_NOARGS},
     {"writable", (PyCFunction)stringio_writable, METH_NOARGS},
+
+    {"__getstate__", (PyCFunction)stringio_getstate, METH_NOARGS},
+    {"__setstate__", (PyCFunction)stringio_setstate, METH_O},
     {NULL, NULL}        /* sentinel */
 };