A novel radiation grafted ETFE based proton conducting membrane was prepared by double irradiation grafting of two different monomers. The intrinsic oxidative stability of the ETFE-g-poly(styrene sulfonic acid-co-divinylbenzene) membrane was improved by reducing the gas crossover through incorporation of polymethacrylonitrile (PMAN) containing the strong polar nitrile group. A fuel cell test was carried out at 80 °C under constant current density of 500 mA cm⁻² for a time exceeding 1′900 h. The incorporation of PMAN considerably improves the interfacial properties of the membrane-electrode assembly. No significant change in the membrane hydrogen crossover and performance over the testing time was observed, except for a measured decrease in the membrane ohmic resistance after 1'000 h. The combination of the double irradiation induced grafting with the use of the PMAN as gas barrier in addition to its chelating abilities (e. g. Ce³⁺) offers a promising strategy to develop more durable membranes for fuel cells.