diff --git a/Doc/c-api/memory.rst b/Doc/c-api/memory.rst
index b79b7e4..9b42900 100644
--- a/Doc/c-api/memory.rst
+++ b/Doc/c-api/memory.rst
@@ -440,8 +440,9 @@
 
    Setup hooks to detect bugs in the Python memory allocator functions.
 
-   Newly allocated memory is filled with the byte ``0xCB``, freed memory is
-   filled with the byte ``0xDB``.
+   Newly allocated memory is filled with the byte ``0xCD`` (``CLEANBYTE``),
+   freed memory is filled with the byte ``0xDD`` (``DEADBYTE``). Memory blocks
+   are surrounded by "forbidden bytes" (``FORBIDDENBYTE``: byte ``0xFD``).
 
    Runtime checks:
 
@@ -471,6 +472,12 @@
       if the GIL is held when functions of :c:data:`PYMEM_DOMAIN_OBJ` and
       :c:data:`PYMEM_DOMAIN_MEM` domains are called.
 
+   .. versionchanged:: 3.7.3
+      Byte patterns ``0xCB`` (``CLEANBYTE``), ``0xDB`` (``DEADBYTE``) and
+      ``0xFB`` (``FORBIDDENBYTE``) have been replaced with ``0xCD``, ``0xDD``
+      and ``0xFD`` to use the same values than Windows CRT debug ``malloc()``
+      and ``free()``.
+
 
 .. _pymalloc:
 
diff --git a/Include/internal/mem.h b/Include/internal/mem.h
index a731e30..5896e4a 100644
--- a/Include/internal/mem.h
+++ b/Include/internal/mem.h
@@ -145,6 +145,30 @@
 
 #define _PyGC_generation0 _PyRuntime.gc.generation0
 
+/* Heuristic checking if a pointer value is newly allocated
+   (uninitialized) or newly freed. The pointer is not dereferenced, only the
+   pointer value is checked.
+
+   The heuristic relies on the debug hooks on Python memory allocators which
+   fills newly allocated memory with CLEANBYTE (0xCD) and newly freed memory
+   with DEADBYTE (0xDD). Detect also "untouchable bytes" marked
+   with FORBIDDENBYTE (0xFD). */
+static inline int _PyMem_IsPtrFreed(void *ptr)
+{
+    uintptr_t value = (uintptr_t)ptr;
+#if SIZEOF_VOID_P == 8
+    return (value == (uintptr_t)0xCDCDCDCDCDCDCDCD
+            || value == (uintptr_t)0xDDDDDDDDDDDDDDDD
+            || value == (uintptr_t)0xFDFDFDFDFDFDFDFD);
+#elif SIZEOF_VOID_P == 4
+    return (value == (uintptr_t)0xCDCDCDCD
+            || value == (uintptr_t)0xDDDDDDDD
+            || value == (uintptr_t)0xFDFDFDFD);
+#else
+#  error "unknown pointer size"
+#endif
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/Include/pymem.h b/Include/pymem.h
index ef6e0bb..458a648 100644
--- a/Include/pymem.h
+++ b/Include/pymem.h
@@ -55,8 +55,6 @@
 PyAPI_FUNC(PyObject*) _PyTraceMalloc_GetTraceback(
     unsigned int domain,
     uintptr_t ptr);
-
-PyAPI_FUNC(int) _PyMem_IsFreed(void *ptr, size_t size);
 #endif   /* !defined(Py_LIMITED_API) */
 
 
diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index 65e0795..d94ee02 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -462,11 +462,11 @@
                  r"    The [0-9] pad bytes at p-[0-9] are FORBIDDENBYTE, as expected.\n"
                  r"    The [0-9] pad bytes at tail={ptr} are not all FORBIDDENBYTE \(0x[0-9a-f]{{2}}\):\n"
                  r"        at tail\+0: 0x78 \*\*\* OUCH\n"
-                 r"        at tail\+1: 0xfb\n"
-                 r"        at tail\+2: 0xfb\n"
+                 r"        at tail\+1: 0xfd\n"
+                 r"        at tail\+2: 0xfd\n"
                  r"        .*\n"
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
-                 r"    Data at p: cb cb cb .*\n"
+                 r"    Data at p: cd cd cd .*\n"
                  r"\n"
                  r"Enable tracemalloc to get the memory block allocation traceback\n"
                  r"\n"
@@ -482,7 +482,7 @@
                  r"    The [0-9] pad bytes at p-[0-9] are FORBIDDENBYTE, as expected.\n"
                  r"    The [0-9] pad bytes at tail={ptr} are FORBIDDENBYTE, as expected.\n"
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
-                 r"    Data at p: cb cb cb .*\n"
+                 r"    Data at p: cd cd cd .*\n"
                  r"\n"
                  r"Enable tracemalloc to get the memory block allocation traceback\n"
                  r"\n"
@@ -508,6 +508,29 @@
         code = 'import _testcapi; _testcapi.pyobject_malloc_without_gil()'
         self.check_malloc_without_gil(code)
 
+    def check_pyobject_is_freed(self, func):
+        code = textwrap.dedent('''
+            import gc, os, sys, _testcapi
+            # Disable the GC to avoid crash on GC collection
+            gc.disable()
+            obj = _testcapi.{func}()
+            error = (_testcapi.pyobject_is_freed(obj) == False)
+            # Exit immediately to avoid a crash while deallocating
+            # the invalid object
+            os._exit(int(error))
+        ''')
+        code = code.format(func=func)
+        assert_python_ok('-c', code, PYTHONMALLOC=self.PYTHONMALLOC)
+
+    def test_pyobject_is_freed_uninitialized(self):
+        self.check_pyobject_is_freed('pyobject_uninitialized')
+
+    def test_pyobject_is_freed_forbidden_bytes(self):
+        self.check_pyobject_is_freed('pyobject_forbidden_bytes')
+
+    def test_pyobject_is_freed_free(self):
+        self.check_pyobject_is_freed('pyobject_freed')
+
 
 class PyMemMallocDebugTests(PyMemDebugTests):
     PYTHONMALLOC = 'malloc_debug'
diff --git a/Misc/NEWS.d/next/C API/2019-04-11-12-20-35.bpo-36389.P9QFoP.rst b/Misc/NEWS.d/next/C API/2019-04-11-12-20-35.bpo-36389.P9QFoP.rst
new file mode 100644
index 0000000..f2b507a
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2019-04-11-12-20-35.bpo-36389.P9QFoP.rst
@@ -0,0 +1,5 @@
+Change the value of ``CLEANBYTE``, ``DEADDYTE`` and ``FORBIDDENBYTE`` internal
+constants used by debug hooks on Python memory allocators
+(:c:func:`PyMem_SetupDebugHooks` function). Byte patterns ``0xCB``, ``0xDB``
+and ``0xFB`` have been replaced with ``0xCD``, ``0xDD`` and ``0xFD`` to use the
+same values than Windows CRT debug ``malloc()`` and ``free()``.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 1e33ca8..b864f92 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -4228,6 +4228,59 @@
 
 
 static PyObject*
+pyobject_is_freed(PyObject *self, PyObject *op)
+{
+    int res = _PyObject_IsFreed(op);
+    return PyBool_FromLong(res);
+}
+
+
+static PyObject*
+pyobject_uninitialized(PyObject *self, PyObject *args)
+{
+    PyObject *op = (PyObject *)PyObject_Malloc(sizeof(PyObject));
+    if (op == NULL) {
+        return NULL;
+    }
+    /* Initialize reference count to avoid early crash in ceval or GC */
+    Py_REFCNT(op) = 1;
+    /* object fields like ob_type are uninitialized! */
+    return op;
+}
+
+
+static PyObject*
+pyobject_forbidden_bytes(PyObject *self, PyObject *args)
+{
+    /* Allocate an incomplete PyObject structure: truncate 'ob_type' field */
+    PyObject *op = (PyObject *)PyObject_Malloc(offsetof(PyObject, ob_type));
+    if (op == NULL) {
+        return NULL;
+    }
+    /* Initialize reference count to avoid early crash in ceval or GC */
+    Py_REFCNT(op) = 1;
+    /* ob_type field is after the memory block: part of "forbidden bytes"
+       when using debug hooks on memory allocatrs! */
+    return op;
+}
+
+
+static PyObject*
+pyobject_freed(PyObject *self, PyObject *args)
+{
+    PyObject *op = _PyObject_CallNoArg((PyObject *)&PyBaseObject_Type);
+    if (op == NULL) {
+        return NULL;
+    }
+    Py_TYPE(op)->tp_dealloc(op);
+    /* Reset reference count to avoid early crash in ceval or GC */
+    Py_REFCNT(op) = 1;
+    /* object memory is freed! */
+    return op;
+}
+
+
+static PyObject*
 pyobject_malloc_without_gil(PyObject *self, PyObject *args)
 {
     char *buffer;
@@ -4788,6 +4841,10 @@
     {"pymem_api_misuse", pymem_api_misuse, METH_NOARGS},
     {"pymem_malloc_without_gil", pymem_malloc_without_gil, METH_NOARGS},
     {"pymem_getallocatorsname", test_pymem_getallocatorsname, METH_NOARGS},
+    {"pyobject_is_freed", (PyCFunction)(void(*)(void))pyobject_is_freed, METH_O},
+    {"pyobject_uninitialized", pyobject_uninitialized, METH_NOARGS},
+    {"pyobject_forbidden_bytes", pyobject_forbidden_bytes, METH_NOARGS},
+    {"pyobject_freed", pyobject_freed, METH_NOARGS},
     {"pyobject_malloc_without_gil", pyobject_malloc_without_gil, METH_NOARGS},
     {"tracemalloc_track", tracemalloc_track, METH_VARARGS},
     {"tracemalloc_untrack", tracemalloc_untrack, METH_VARARGS},
diff --git a/Objects/object.c b/Objects/object.c
index 138df44..420af94 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -411,28 +411,26 @@
 }
 
 
-/* Heuristic checking if the object memory has been deallocated.
-   Rely on the debug hooks on Python memory allocators which fills the memory
-   with DEADBYTE (0xDB) when memory is deallocated.
+/* Heuristic checking if the object memory is uninitialized or deallocated.
+   Rely on the debug hooks on Python memory allocators:
+   see _PyMem_IsPtrFreed().
 
    The function can be used to prevent segmentation fault on dereferencing
-   pointers like 0xdbdbdbdbdbdbdbdb. Such pointer is very unlikely to be mapped
-   in memory. */
+   pointers like 0xDDDDDDDDDDDDDDDD. */
 int
 _PyObject_IsFreed(PyObject *op)
 {
-    uintptr_t ptr = (uintptr_t)op;
-    if (_PyMem_IsFreed(&ptr, sizeof(ptr))) {
+    if (_PyMem_IsPtrFreed(op) || _PyMem_IsPtrFreed(op->ob_type)) {
         return 1;
     }
-    int freed = _PyMem_IsFreed(&op->ob_type, sizeof(op->ob_type));
-    /* ignore op->ob_ref: the value can have be modified
+    /* ignore op->ob_ref: its value can have be modified
        by Py_INCREF() and Py_DECREF(). */
 #ifdef Py_TRACE_REFS
-    freed &= _PyMem_IsFreed(&op->_ob_next, sizeof(op->_ob_next));
-    freed &= _PyMem_IsFreed(&op->_ob_prev, sizeof(op->_ob_prev));
+    if (_PyMem_IsPtrFreed(op->_ob_next) || _PyMem_IsPtrFreed(op->_ob_prev)) {
+        return 1;
+    }
 #endif
-    return freed;
+    return 0;
 }
 
 
@@ -449,7 +447,7 @@
     if (_PyObject_IsFreed(op)) {
         /* It seems like the object memory has been freed:
            don't access it to prevent a segmentation fault. */
-        fprintf(stderr, "<freed object>\n");
+        fprintf(stderr, "<Freed object>\n");
         return;
     }
 
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 3b0c35b..46e8427 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1946,14 +1946,17 @@
 
 /* Special bytes broadcast into debug memory blocks at appropriate times.
  * Strings of these are unlikely to be valid addresses, floats, ints or
- * 7-bit ASCII.
+ * 7-bit ASCII. If modified, _PyMem_IsPtrFreed() should be updated as well.
+ *
+ * Byte patterns 0xCB, 0xBB and 0xFB have been replaced with 0xCD, 0xDD and
+ * 0xFD to use the same values than Windows CRT debug malloc() and free().
  */
 #undef CLEANBYTE
 #undef DEADBYTE
 #undef FORBIDDENBYTE
-#define CLEANBYTE      0xCB    /* clean (newly allocated) memory */
-#define DEADBYTE       0xDB    /* dead (newly freed) memory */
-#define FORBIDDENBYTE  0xFB    /* untouchable bytes at each end of a block */
+#define CLEANBYTE      0xCD    /* clean (newly allocated) memory */
+#define DEADBYTE       0xDD    /* dead (newly freed) memory */
+#define FORBIDDENBYTE  0xFD    /* untouchable bytes at each end of a block */
 
 static size_t serialno = 0;     /* incremented on each debug {m,re}alloc */
 
@@ -2091,22 +2094,6 @@
 }
 
 
-/* Heuristic checking if the memory has been freed. Rely on the debug hooks on
-   Python memory allocators which fills the memory with DEADBYTE (0xDB) when
-   memory is deallocated. */
-int
-_PyMem_IsFreed(void *ptr, size_t size)
-{
-    unsigned char *bytes = ptr;
-    for (size_t i=0; i < size; i++) {
-        if (bytes[i] != DEADBYTE) {
-            return 0;
-        }
-    }
-    return 1;
-}
-
-
 /* The debug free first checks the 2*SST bytes on each end for sanity (in
    particular, that the FORBIDDENBYTEs with the api ID are still intact).
    Then fills the original bytes with DEADBYTE.