The durability and degradation behavior of the Nafion NR111 proton exchange membranes (PEMs) is investigated in detail under various mechanical, chemical and polarization conditions. It was found that the fatigue strength or the safety limit of the cyclic stress for Nafion NR111 membrane is ∼1.5 MPa that is 1/10 of the tensile strength of the membrane. The cyclic stress and dimensional change (or strain) induced by the water uptake can be substantial and are the main causes for the mechanical degradation and failure of the membrane. For example, in the case of RH cycling of water soaked state to 25% RH state, the cyclic stress of the Nafion membrane was as high as 2.23 MPa and the dimensional change was ∼11%. Both FTIR and NMR analysis indicate that the decomposition of the Nafion polymer in the H 2O 2 solution in the presence of trace Fe, Cr and Ni ions started from the ends of the main chain, resulting in the loss of the repeat units and the formation of voids and pinholes in the membrane. The high degradation rate of the membrane at the open circuit voltage most likely results from the attack of H 2O 2 formed between O 2 and H 2 crossovered through the membrane.