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 I wherein R1 is C1-C10 alkyl, C3-C12 cycloalkyl, C6-C14 aryl, allyl, alkenyl, alkoxy, halo, or cyano; Ar1 and Ar2 are each independently C6-C20 aromatic radicals, or Ar1 and Ar2, taken together with an intervening carbon atom, form a bicyclic C6-C20 aromatic radical or a tricyclic C6-C20 aromatic radical; M is H, a metal cation, a non-metallic inorganic cation, an organic cation or a mixture thereof; and a is 0 or an integer from 1 to 4.