diff -r 483096ef1cf6 Lib/_pyio.py
--- a/Lib/_pyio.py	Wed Jan 29 11:45:31 2014 +0200
+++ b/Lib/_pyio.py	Wed Jan 29 12:16:31 2014 +0200
@@ -2050,7 +2050,14 @@
     """
 
     def __init__(self, initial_value="", newline="\n"):
-        super(StringIO, self).__init__(BytesIO(),
+        if initial_value is not None:
+            if not isinstance(initial_value, str):
+                raise TypeError("initial_value must be str or None, not {0}"
+                                .format(type(initial_value).__name__))
+            initial_value = initial_value.encode('utf-8', 'surrogatepass')
+        else:
+            initial_value = b''
+        super(StringIO, self).__init__(BytesIO(initial_value),
                                        encoding="utf-8",
                                        errors="surrogatepass",
                                        newline=newline)
@@ -2058,12 +2065,6 @@
         # C version, even under Windows.
         if newline is None:
             self._writetranslate = False
-        if initial_value is not None:
-            if not isinstance(initial_value, str):
-                raise TypeError("initial_value must be str or None, not {0}"
-                                .format(type(initial_value).__name__))
-            self.write(initial_value)
-            self.seek(0)
 
     def getvalue(self):
         self.flush()
diff -r 483096ef1cf6 Lib/test/test_memoryio.py
--- a/Lib/test/test_memoryio.py	Wed Jan 29 11:45:31 2014 +0200
+++ b/Lib/test/test_memoryio.py	Wed Jan 29 12:16:31 2014 +0200
@@ -539,53 +539,81 @@
     def test_newline_none(self):
         # newline=None
         memio = self.ioclass("a\nb\r\nc\rd", newline=None)
+        #self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
         self.assertEqual(list(memio), ["a\n", "b\n", "c\n", "d"])
         memio.seek(0)
         self.assertEqual(memio.read(1), "a")
         self.assertEqual(memio.read(2), "\nb")
         self.assertEqual(memio.read(2), "\nc")
         self.assertEqual(memio.read(1), "\n")
+
         memio = self.ioclass(newline=None)
         self.assertEqual(2, memio.write("a\n"))
         self.assertEqual(3, memio.write("b\r\n"))
         self.assertEqual(3, memio.write("c\rd"))
+        #self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
         memio.seek(0)
         self.assertEqual(memio.read(), "a\nb\nc\nd")
+
         memio = self.ioclass("a\r\nb", newline=None)
         self.assertEqual(memio.read(3), "a\nb")
 
     def test_newline_empty(self):
         # newline=""
         memio = self.ioclass("a\nb\r\nc\rd", newline="")
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
         self.assertEqual(list(memio), ["a\n", "b\r\n", "c\r", "d"])
         memio.seek(0)
         self.assertEqual(memio.read(4), "a\nb\r")
         self.assertEqual(memio.read(2), "\nc")
         self.assertEqual(memio.read(1), "\r")
+
         memio = self.ioclass(newline="")
         self.assertEqual(2, memio.write("a\n"))
         self.assertEqual(2, memio.write("b\r"))
         self.assertEqual(2, memio.write("\nc"))
         self.assertEqual(2, memio.write("\rd"))
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
         memio.seek(0)
         self.assertEqual(list(memio), ["a\n", "b\r\n", "c\r", "d"])
 
     def test_newline_lf(self):
         # newline="\n"
-        memio = self.ioclass("a\nb\r\nc\rd")
+        memio = self.ioclass("a\nb\r\nc\rd", newline="\n")
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
+        self.assertEqual(list(memio), ["a\n", "b\r\n", "c\rd"])
+
+        memio = self.ioclass(newline="\n")
+        self.assertEqual(memio.write("a\nb\r\nc\rd"), 8)
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
+        memio.seek(0)
         self.assertEqual(list(memio), ["a\n", "b\r\n", "c\rd"])
 
     def test_newline_cr(self):
         # newline="\r"
         memio = self.ioclass("a\nb\r\nc\rd", newline="\r")
-        self.assertEqual(memio.read(), "a\rb\r\rc\rd")
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
+        self.assertEqual(memio.read(), "a\nb\r\nc\rd")
+        memio.seek(0)
+        self.assertEqual(list(memio),['a\nb\r', '\nc\r', 'd'])
+
+        memio = self.ioclass(newline="\r")
+        self.assertEqual(memio.write("a\nb\r\nc\rd"), 8)
+        self.assertEqual(memio.getvalue(), "a\rb\r\rc\rd")
         memio.seek(0)
         self.assertEqual(list(memio), ["a\r", "b\r", "\r", "c\r", "d"])
 
     def test_newline_crlf(self):
         # newline="\r\n"
         memio = self.ioclass("a\nb\r\nc\rd", newline="\r\n")
-        self.assertEqual(memio.read(), "a\r\nb\r\r\nc\rd")
+        self.assertEqual(memio.getvalue(), "a\nb\r\nc\rd")
+        self.assertEqual(memio.read(), "a\nb\r\nc\rd")
+        memio.seek(0)
+        self.assertEqual(list(memio), ['a\nb\r\n', 'c\rd'])
+
+        memio = self.ioclass(newline="\r\n")
+        self.assertEqual(memio.write("a\nb\r\nc\rd"), 8)
+        self.assertEqual(memio.getvalue(), "a\r\nb\r\r\nc\rd")
         memio.seek(0)
         self.assertEqual(list(memio), ["a\r\n", "b\r\r\n", "c\rd"])
 
diff -r 483096ef1cf6 Modules/_io/stringio.c
--- a/Modules/_io/stringio.c	Wed Jan 29 11:45:31 2014 +0200
+++ b/Modules/_io/stringio.c	Wed Jan 29 12:16:31 2014 +0200
@@ -735,14 +735,12 @@
     else
         value_len = 0;
     if (value_len > 0) {
-        /* This is a heuristic, for newline translation might change
-           the string length. */
-        if (resize_buffer(self, 0) < 0)
+        if (resize_buffer(self, value_len) < 0)
             return -1;
+        if (!PyUnicode_AsUCS4(value, self->buf, value_len, 0))
+            return -1;
+        self->string_size = self->pos = value_len;
         self->state = STATE_REALIZED;
-        self->pos = 0;
-        if (write_str(self, value) < 0)
-            return -1;
     }
     else {
         /* Empty stringio object, we can start by accumulating */