Poly(tetrafluoroethylene-co-haxafluoropropylene) (FEP)-g-styrene-acrylic acid and its sulfonated derivative membranes were prepared by graft copolymerization of styrene-acrylic acid onto FEP by using preirradiation of gamma-ray technique followed by sulfonation. The physiochemical properties such as ion exchange capacity, water uptake, ionic resistance of the grafted membranes, and their sulfonated derivatives were studied as a function of degree of grafting. These membranes on sulfonation gave acid base, indicating property. The membranes gave yellow color in acidic medium and purple color in alkali medium. (C) 2004 Wiley Periodicals, Inc.