Index: Lib/test/test_memoryio.py
===================================================================
--- Lib/test/test_memoryio.py	(revision 73313)
+++ Lib/test/test_memoryio.py	(working copy)
@@ -9,7 +9,56 @@
 import io
 import _pyio as pyio
 import sys
+import functools
 
+#XXX: Should check_if_leaking and LeakTestMeta be moved to test.support?
+
+def check_if_leaking(testfunc):
+    """Decorator for checking reference leak regressions.
+    Based roughly on the code in regrtest.
+    """
+    @functools.wraps(testfunc)
+    @unittest.skipIf(not hasattr(sys, "gettotalrefcount"),
+                     "Reference leaks tracking requires debug build")
+    def wrapper(*args, **kwargs):
+        repcount = 8
+        nwarmup = 3
+        deltas = []
+        for i in range(repcount):
+            rc = sys.gettotalrefcount()
+            testfunc(*args, **kwargs)
+            support.gc_collect()
+            if i >= nwarmup:
+                deltas.append(sys.gettotalrefcount() - rc - 2)
+        if any(deltas):
+            raise AssertionError(
+                "{0} leaked: {1}".format(testfunc.__name__, deltas))
+    return wrapper
+
+class LeakTestMeta(type):
+    """Metaclass for adding reference leak checking to all the test methods
+    of a class.
+
+    This particularly useful for test cases composed uniquely of fine-grained
+    reference leak tests.
+    """
+
+    def __new__(cls, name, bases, dict):
+        for attribute in dict:
+            if (attribute.startswith("test") and
+                hasattr(dict[attribute], "__call__")):
+                dict[attribute] = check_if_leaking(dict[attribute])
+        return type(name, bases, dict)
+
+
+class RefleakTestMixin(metaclass=LeakTestMeta):
+
+    def test_instance_dict_leak(self):
+        # Issue #6242
+        memio = self.ioclass()
+        memio.foo = 1
+
+
 class MemorySeekTestMixin:
 
     def testInit(self):
@@ -339,7 +388,8 @@
         self.assertEqual(test2(), buf)
 
 
-class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
+class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, RefleakTestMixin,
+                    unittest.TestCase):
 
     UnsupportedOperation = pyio.UnsupportedOperation
 
@@ -417,7 +467,8 @@
         self.assertEqual(memio.getvalue(), buf)
 
 
-class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
+class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin, RefleakTestMixin,
+                     unittest.TestCase):
     buftype = str
     ioclass = pyio.StringIO
     UnsupportedOperation = pyio.UnsupportedOperation
Index: Modules/_io/bytesio.c
===================================================================
--- Modules/_io/bytesio.c	(revision 73313)
+++ Modules/_io/bytesio.c	(working copy)
@@ -606,14 +606,31 @@
     Py_RETURN_NONE;
 }
 
+static int
+bytesio_traverse(BytesIOObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->dict);
+    return 0;
+}
+
+static int
+bytesio_clear(BytesIOObject *self)
+{
+    Py_CLEAR(self->dict);
+    return 0;
+}
+
 static void
 bytesio_dealloc(BytesIOObject *self)
 {
-    if (self->buf != NULL) {
+    _PyObject_GC_UNTRACK(self);
+    if (self->buf) {
         PyMem_Free(self->buf);
         self->buf = NULL;
     }
-    Py_TYPE(self)->tp_clear((PyObject *)self);
+    Py_CLEAR(self->dict);
+    if (self->weakreflist != NULL)
+        PyObject_ClearWeakRefs((PyObject *)self);
     Py_TYPE(self)->tp_free(self);
 }
 
@@ -663,24 +680,6 @@
     return 0;
 }
 
-static int
-bytesio_traverse(BytesIOObject *self, visitproc visit, void *arg)
-{
-    Py_VISIT(self->dict);
-    Py_VISIT(self->weakreflist);
-    return 0;
-}
-
-static int
-bytesio_clear(BytesIOObject *self)
-{
-    Py_CLEAR(self->dict);
-    if (self->weakreflist != NULL)
-        PyObject_ClearWeakRefs((PyObject *)self);
-    return 0;
-}
-
-
 static PyGetSetDef bytesio_getsetlist[] = {
     {"closed",  (getter)bytesio_get_closed, NULL,
      "True if the file is closed."},
Index: Modules/_io/stringio.c
===================================================================
--- Modules/_io/stringio.c	(revision 73313)
+++ Modules/_io/stringio.c	(working copy)
@@ -509,11 +509,15 @@
 stringio_dealloc(StringIOObject *self)
 {
     _PyObject_GC_UNTRACK(self);
+    self->ok = 0;
+    if (self->buf) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
     Py_CLEAR(self->readnl);
     Py_CLEAR(self->writenl);
     Py_CLEAR(self->decoder);
-    if (self->buf)
-        PyMem_Free(self->buf);
+    Py_CLEAR(self->dict);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
     Py_TYPE(self)->tp_free(self);