A series of poly(arylene ether ether nitrile)s with different chain lengths of the alkylsulfonates (SPAEEN-x: x refers number of the methylene units) are successfully synthesized for fuel cell applications. The polymers produced flexible and transparent membranes by solvent casting. The resulting membranes display a high thermal stability, oxidative stability, and higher proton conductivity than that of Nafion 117 at 80 degrees C and 95% relative humidity (RH). Furthermore, the SPAEEN-12 with the longest alkylsulfonated side chain exhibits a higher proton conductivity at 30% RH than that of SPAEEN-6 despite the lower IEC value, which indicates that the introduction of longer alkylsufonated side chains to the polymer main chain induces an efficient proton conduction by the formation of a well-developed phase-separated morphology. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 21-29