concentrated alkali metal hydroxide substantially free of alkali metal halide and other impurities is produced by the electrolysis of an alkali metal halide solution in an electrolytic cell having a dimensionally stable anode and a metal cathode separated by an electrically conductive stable selectively permeable hydrated cation ion - exchange membrane film of a fluorinated copolymer having pendant sulfonic acid groups or derivatives of such groups . the membrane film is capable of use at high temperatures and under severely corrosive chemical conditions for extended periods without degradation .