This paper examines damage to the cathode catalyst layer of a polymer electrolyte membrane (PEM) fuel cell caused by restriction of hydrogen access to a portion of the anode catalyst layer. The results show severe damage to the majority of the cathode catalyst behind the obstruction after 100 h of operation. A portion of the cathode catalyst layer near the outside edge of the obstruction remains undamaged. (c) 2006 The Electrochemical Society.