We present a durability test of polymer electrolyte membrane fuel cell (PEMFC) in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly. ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H2O2, CO2, SO, SO2, H2SO2, and H2SO3. A kinetic model is presented assuming that the H-2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H-2 gas. (c) 2006 The Electrochemical Society.