diff --git a/sys/xtimer/xtimer.c b/sys/xtimer/xtimer.c
index c038b1e3d9..787bf7d702 100644
--- a/sys/xtimer/xtimer.c
+++ b/sys/xtimer/xtimer.c
@@ -45,7 +45,7 @@
 typedef struct {
     mutex_t *mutex;
     thread_t *thread;
-    int timeout;
+    volatile int timeout;
 } mutex_thread_t;
 
 static void _callback_unlock_mutex(void* arg)