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 Nation NR111 membrane is similar to 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 Nation membrane was as high as 2.23 MPa and the dimensional change was similar to 11%. Both FTIR and NMR analysis indicate that the decomposition of the Nation polymer in the H2O2 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 H2O2 formed between O-2 and H-2 crossovered through the membrane. (C) 2007 Elsevier B.V. All rights reserved.