diff --git a/lib/std/os/win32/process.c3 b/lib/std/os/win32/process.c3
index dd6738b60..9788aea29 100644
--- a/lib/std/os/win32/process.c3
+++ b/lib/std/os/win32/process.c3
@@ -48,14 +48,14 @@ const UNDNAME_COMPLETE = 0x0000;
 
 extern fn void initializeCriticalSection(Win32_CRITICAL_SECTION* section) @extern("InitializeCriticalSection");
 extern fn void deleteCriticalSection(Win32_CRITICAL_SECTION* section) @extern("DeleteCriticalSection");
-extern fn Win32_HANDLE createMutex(void*, bool, void*) @extern("CreateMutexA");
+extern fn Win32_HANDLE createMutex(void*, Win32_BOOL, void*) @extern("CreateMutexA");
 extern fn Win32_BOOL releaseMutex(Win32_HANDLE) @extern("ReleaseMutex");
 extern fn void enterCriticalSection(Win32_CRITICAL_SECTION* section) @extern("EnterCriticalSection");
 extern fn void leaveCriticalSection(Win32_CRITICAL_SECTION* section) @extern("LeaveCriticalSection");
 extern fn Win32_BOOL tryEnterCriticalSection(Win32_CRITICAL_SECTION* section) @extern("TryEnterCriticalSection");
 extern fn uint waitForSingleObject(Win32_HANDLE, uint milliseconds) @extern("WaitForSingleObject");
 extern fn void sleep(uint ms) @extern("Sleep");
-extern fn uint waitForMultipleObjects(uint count, Win32_HANDLE* handles, bool wait_all, uint ms) @extern("WaitForMultipleObjects");
+extern fn uint waitForMultipleObjects(uint count, Win32_HANDLE* handles, Win32_BOOL wait_all, uint ms) @extern("WaitForMultipleObjects");
 extern fn Win32_BOOL resetEvent(Win32_HANDLE event) @extern("ResetEvent");
 extern fn Win32_BOOL setEvent(Win32_HANDLE handle) @extern("SetEvent");
 extern fn long interlockedCompareExchange(int* dest, int exchange, int comperand) @extern("InterlockedCompareExchange");
diff --git a/lib/std/threads/os/thread_win32.c3 b/lib/std/threads/os/thread_win32.c3
index b7e08dabb..75f473ffe 100644
--- a/lib/std/threads/os/thread_win32.c3
+++ b/lib/std/threads/os/thread_win32.c3
@@ -48,7 +48,7 @@ fn void! NativeMutex.init(&mtx, MutexType type)
 		win32::initializeCriticalSection(&(mtx.critical_section));
 		return;
 	}
-	if (!(mtx.handle = win32::createMutex(null, false, null))) return ThreadFault.INIT_FAILED?;
+	if (!(mtx.handle = win32::createMutex(null, 0, null))) return ThreadFault.INIT_FAILED?;
 }
 
 fn void! NativeMutex.destroy(&mtx)
@@ -208,7 +208,7 @@ fn void! timedwait(NativeConditionVariable* cond, NativeMutex* mtx, uint timeout
 
 	mtx.unlock()!;
 
-	uint result = win32::waitForMultipleObjects(2, &cond.events, false, timeout);
+	uint result = win32::waitForMultipleObjects(2, &cond.events, 0, timeout);
 	switch (result)
 	{
 		case win32::WAIT_TIMEOUT:
diff --git a/releasenotes.md b/releasenotes.md
index 65c230010..f2f4d6271 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -23,6 +23,7 @@
 
 ### Stdlib changes
 - Add 'zstr' variants for `string::new_format` / `string::tformat`.
+- Fix mutex and wait signatures for Win32.
 
 ## 0.5.5 Change list