Proton exchange membranes for high temperature fuel cell applications were obtained via graft copolymerizations on commercial poly(ethylene‐alt‐tetrafluoroethylene) (ETFE) films. ETFE was activated by electron beam treatment, and the hydrophilic monomers acrylic acid and hydroxyethyl methacrylate were grafted to ETFE. The maximum grafting level was 620%. The grafted membranes were doped with the proton carrier phosphoric acid leading to a maximum doping level of 450%. Stress–strain measurements indicate good mechanical stability of the composite membranes. The polymer–acid membranes were tested in a H2/O2 fuel cell. At a current density of 200 mA · cm−2 a stable power density of 113 mW · cm−2 was determined at 120 °C. The results show that high phosphoric acid contents associated with high proton conductivity may be obtained with a copolymer containing acrylic acid.