Proton-exchange membranes were synthesised by electron beam irradiation of poly(vinylidenefluoride) (PVDF) films (80 mu m) followed by styrene graft polymerisation and sulfonation. Physical and electrochemical properties of the membranes were investigated. The membranes were tested as electrolytes in fuel cell conditions (humidified H-2/O-2, T = 20-60 degrees C, ambient pressure); emfs of ca. 0.85 V were observed during operation for > 150 h. The conductivity of PVDF-g-pssa (30% graft) electrolyte membranes at 23 degrees C is 0.03 S cm(-1) with an overall cell resistance of 6.5 ohm cm(2) under fuel cell conditions.