Sulfonated block copolymer suitable for use as proton exchange membranes for fuel cells comprise sulfonated polyaryletherketone blocks and polyethersulfone blocks. The sulfonated polyaryletherketone blocks comprise structural units of formula ##STR00001## the polyethersulfone blocks comprising structural units of formula ##STR00002## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently C.sub.1-C.sub.10 alkyl, C.sub.3-C.sub.12 cycloalkyl, C.sub.6-C.sub.14 aryl, allyl, alkenyl, alkoxy,halo, or cyano; Z and Z.sup.1 are independently a direct bond or O, S, (CH.sub.2).sub.r; (CF.sub.2).sub.r, C(CH.sub.3).sub.2, C(CF.sub.3).sub.2, or a combination thereof; M is H, a metal cation, a non-metallic inorganic cation, an organic cation or a mixture thereof a is 0 or an integer from 1 to 3; b, c and d are independently 0 or an integer from 1 to 4; m and n are independently 0 or 1; and r is an integer from 1 to 5.