The mechanical durability of sulfonated poly (phenylene) (SPP) membrane, used for polymer electrolyte fuel cells (PEFCs), is evaluated by the United States Department of Energy (USDOE) stress protocol involving wet-dry cycling, and the degradation is analyzed specifically by comparing with sulfonated poly(arylene ether ketone) (SPK) membrane. Initially, the SPP membrane exhibits 2-fold higher stiffness and 50% lower dimensional change ratio than the SPK membrane. In durability cycling, the SPP membrane lasts more than 20,000 wet-dry cycles without mechanical failure, which is more than 5-fold better durability than that for the SPK membrane. Higher mechanical strength and lower dimensional change can reduce both irreversible membrane deformation and mechanical stress attributed to the membrane swelling and shrinking. In post-test analyses, the SPP membrane is found to rupture in the peripheral region of the membrane electrode assemblies. The SPP membrane maintains only 10% of the elongation at break in the peripheral region but 50% in the electrode region, compared with the pristine condition. It is most likely that the membrane deteriorates in the peripheral region due to stress concentration by cell compression and membrane deformation during wet-dry cycling. (185 words) (C) The Author(s) 2017. Published by ECS. All rights reserved.