[en] Preparation of a graft copolymer membrane prepared by co-grafting a mixture of monomers consisting essentially of acyloxystyrene and/or hydroxystyrene, a polyene compound and at least one member selected from the group consisting of acrylic acid, methacrylic acid and esters of these acids onto a hydrocarbon type of fluorine-containing type polymer by means of ionizing radiation is described. The graft copolymer membrane exhibits improved mechanical strength and reduced electrical resistance and provides an excellent ion-exchange membrane