Polymer electrolyte membranes (PEMs) containing alkylsulfonic acid grafts can be prepared by radiation-induced graft copolymerization of methyl acrylate (MA) and methyl methacrylate (MMA) into a poly(ethylene-co-tetrafluoroethylene) film followed by sulfonation the MA units in the copolymer grafts using an equimolar complex of chlorosulfonic acid and 1,4-dioxane (CISO3H-Complex). PEMs with MA/MMA copolymer grafts that are 33%-79%,) MA units were prepared by preirradiation with a dose of 20 kGy and grafting in bulk comonomers at 60 degrees C. The grafted films are treated with CISO3H-Complex to obtain PEMs with ion exchange capacity of 0.36-0.81 mmol/g (sulfonation degrees of 20%-40%) and proton conductivity of 0.04-0.065 S/cm. These values can be controlled by changing the MA content the sulfonation occurring at an alpha-carbonyl carbon. The PEMs with higher MMA content showed higher durability in water (80 degrees C) and under oxidative conditions (3% H2O2) at 60 degrees C. This is because the PMMA grafts in the PEMs have no proton at an a-carbonyl carbon, which is considered to be a trigger of the degradation of grafting polymers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 231-237, 2009