Sulfonated block copolymers that may be used as membranes for fuel cells include sulfonated polyaryletherketone blocks and lightly sulfonated polyethersulfone blocks.The sulfonated polyaryletherketone blocks include structural units of formula ##STR00001## and the lightly sulfonated polyethersulfone blocks include structural units of formula ##STR00002## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 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, Z.sup.1 and Z.sup.2 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 and e are independently 0 or an integer from 1 to 3; b, c, d, and f are independently 0 or an integer from 1 to 4; m, n and p are independently 0 or 1; and r is an integer from 1 to 5.