diff --git a/src/call_once.cpp b/src/call_once.cpp
index 5744347..2b981b6 100644
--- a/src/call_once.cpp
+++ b/src/call_once.cpp
@@ -28,7 +28,12 @@ static constinit __libcpp_mutex_t mut  = _LIBCPP_MUTEX_INITIALIZER;
 static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
 #endif
 
-void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
+#ifdef _LIBCPP_ABI_MICROSOFT
+void __call_once(volatile std::atomic<once_flag::_State_type>& flag, void* arg, void (*func)(void*))
+#else
+void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*))
+#endif
+{
 #if defined(_LIBCPP_HAS_NO_THREADS)
 
   if (flag == once_flag::_Unset) {
@@ -47,16 +52,28 @@ void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(
   if (flag == once_flag::_Unset) {
     auto guard = std::__make_exception_guard([&flag] {
       __libcpp_mutex_lock(&mut);
+#  ifdef _LIBCPP_ABI_MICROSOFT
+      flag.store(once_flag::_Unset);
+#  else
       __libcpp_relaxed_store(&flag, once_flag::_Unset);
+#  endif
       __libcpp_mutex_unlock(&mut);
       __libcpp_condvar_broadcast(&cv);
     });
 
+#  ifdef _LIBCPP_ABI_MICROSOFT
+    flag.store(once_flag::_Pending, memory_order_relaxed);
+#  else
     __libcpp_relaxed_store(&flag, once_flag::_Pending);
+#  endif
     __libcpp_mutex_unlock(&mut);
     func(arg);
     __libcpp_mutex_lock(&mut);
+#  ifdef _LIBCPP_ABI_MICROSOFT
+    flag.store(once_flag::_Complete, memory_order_release);
+#  else
     __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
+#  endif
     __libcpp_mutex_unlock(&mut);
     __libcpp_condvar_broadcast(&cv);
     guard.__complete();
diff --git a/src/mutex.cpp b/src/mutex.cpp
index 2f8504d..26b26b6 100644
--- a/src/mutex.cpp
+++ b/src/mutex.cpp
@@ -12,7 +12,9 @@
 #include <limits>
 #include <mutex>
 
-#include "include/atomic_support.h"
+#if !defined(_LIBCPP_ABI_MICROSOFT)
+#  include "include/atomic_support.h"
+#endif
 
 #if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
 #  pragma comment(lib, "pthread")