A series of multiblock poly(phenylene ether nitrile)s with pendant sulfoalkoxyl side chains have been developed as proton exchange membranes for fuel cells. The membranes were obtained by a solution casting method and exhibited good thermal stability, flexibility, and mechanical strength. The membranes displayed well-developed microphase separation, which largely contributed to their excellent ion conduction ability. One of the new membranes with a low ion exchange capacity of 1.57 mequiv g(-1) showed higher proton conductivity than Nafion 212 over the entire RH range (30-95%). The maximum power output generated in a single cell test reached up to 0.754, 0.640, and 0.414 W cm(-2) at 70 degrees C under 80%, 50%, and 30% RH conditions, respectively. The current density of the membrane obtained at 0.6 V (I-0.6) was as high as 640 mA cm(-2), which was much higher than that of Nafion 212 (375 mA cm(-2) at 30% RH), suggesting its superiority for a more rapid system start-up. Furthermore, the in situ durability test at 50% RH was performed at a constant current loading, and the membrane did not show any significant voltage reduction over the 400 h testing period. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1940-1948