Two incidents of oxygen contamination. in the hospital medical air supply occurred on consecutive weekends. This paper describes those incidents and the investigation that followed. The cause of the contamination was determined to be faulty, check values in some Bird air/oxygen blenders. The investigation of the incidents was hindered because the conventional blender test methods did not detect the problem. Only after monitoring the medical air and oxygen pressures and simulating the real conditions did the cause of the problem surface.