An accelerated degradation test has been conducted to determine the mechanisms affected on the degradation of PEM water electrolysis cells. During the express-test, the cell voltage was steadily increased until a cell short-circuit was detected after approximately 5500 h of operation. Post-mortem analysis permits to conclude that the first mechanism of MEA degradation is a corrosion process in the catalysts layer with simultaneous migration of dissolved platinum to the membrane. It was found as well that the membrane thickness has been reduced by approximately 75% during the test. Membrane thinning is therefore a second degradation mechanism that led to cell failure.