Mechanical endurance and degradation mechanism of Nafion211 proton exchange membrane and ePTFE/PFSA composite PEMs were investigated in this article. It was found that the shrinkage stress caused by the water-uptake is the primary source to induce the mechanical decay. The Nafion211 is more stable when cycle-stress is lower than 1.5 MPa, while the water-uptake generating stress can reach to 2.23 MPa soaked in surroundings with condition of 25% RH and temperature at 25 degrees C. The study also found that the ePTFE/PFSA composite PEMs are more durable than the Nafion membrane mostly due to the lower water-uptake generating shrinkage stress of 0.34-0.4 MPa (25% RH@ 25 degrees C) but not the high yield strength or breaking strength. The PFSA/ePTFE composite membranes can keep stable for more than 5000 cycles, which is about 40% higher than that of the pure Nafion membrane (about 3500 cycles.). For preparing durable proton exchange membrane, it is necessary to improve the proton exchange resin impregnation in the ePTFE matrix. Chemical bonding of the PTFE and the PSFI by modification of the PTFE matrix is also very effective to enhance the mechanical durability of the composite PEMs. (C) 2008 Wiley Periodicals, Inc.