diff --git a/doomsday/libdeng2/include/de/concurrency/waitable.h b/doomsday/libdeng2/include/de/concurrency/waitable.h
index e692c6a2c2..6c32842ef3 100644
--- a/doomsday/libdeng2/include/de/concurrency/waitable.h
+++ b/doomsday/libdeng2/include/de/concurrency/waitable.h
@@ -49,19 +49,19 @@ class DENG2_PUBLIC Waitable
     void reset();
 
     /// Wait until the resource becomes available. Waits indefinitely.
-    void wait();
+    void wait() const;
 
     /// Wait for the specified period of time to secure the
     /// resource.  If timeout occurs, an exception is thrown.
-    void wait(TimeDelta const &timeOut);
+    void wait(TimeDelta const &timeOut) const;
 
     /// Mark the resource as available by incrementing the
     /// semaphore value.
-    void post();
+    void post() const;
 
 private:
     /// Pointer to the internal semaphore data.
-    QSemaphore _semaphore;
+    mutable QSemaphore _semaphore;
 };
 
 }
diff --git a/doomsday/libdeng2/src/concurrency/waitable.cpp b/doomsday/libdeng2/src/concurrency/waitable.cpp
index b9cd0e9851..557c9c8cb7 100644
--- a/doomsday/libdeng2/src/concurrency/waitable.cpp
+++ b/doomsday/libdeng2/src/concurrency/waitable.cpp
@@ -33,12 +33,12 @@ void Waitable::reset()
     _semaphore.tryAcquire(_semaphore.available());
 }
 
-void Waitable::wait()
+void Waitable::wait() const
 {
     wait(0.0);
 }
 
-void Waitable::wait(TimeDelta const &timeOut)
+void Waitable::wait(TimeDelta const &timeOut) const
 {
     if(timeOut <= 0.0)
     {
@@ -55,7 +55,7 @@ void Waitable::wait(TimeDelta const &timeOut)
     }
 }
 
-void Waitable::post()
+void Waitable::post() const
 {
     _semaphore.release();
 }