This paper studies the degradation of Gore (TM) PRIMEA (R) series 5510 catalyst coated membranes with an ePTFE reinforcement layer under open circuit voltage conditions at 90 degrees C, 75% RH. and no backpressure. Scanning electron microscopy (SEM) imaging of cross-sections revealed extensive cathode-side ionomer degradation and the presence of a platinum band. Cumulative fluoride release measurements show more fluoride exiting with the cathode effluent. Furthermore, both anode and cathode cumulative fluoride release plateau after long degradation times. Open circuit voltage was also monitored and the degradation rate was found to decrease after a long duration. It is proposed that all fluoride species are generated from the cathode-side ionomer degradation process and that the fluoride then diffuses to the anode and cathode channels. Further, once the cathode-side ionomer is consumed the degradation reaction slows as the "degradation front" passes through the inert reinforcement layer. This process was modeled using a semi-empirical transient model and compared to experimental results. (C) 2008 Published by Elsevier B.V.