Sulfonated polyethersulfones and polymer compositions comprising sulfonated polyethersulfones and at least one inorganic heteropolyacid are useful as proton exchange membranes for fuel cells. The polyethersulfones include structural units derived from at least one biphenol monomer and a monomer of formula I ##STR00001## wherein X is O, S or NR.sup.1; Y is N or CR.sup.5; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, alkylaryl, substituted alkylaryl, arylalkyl or substituted arylalkyl, or R.sup.3 and R.sup.4, taken together, form a 5- or 6-membered substituted or unsubstituted aliphatic or aromatic ring; R.sup.5 is hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, alkylaryl, substituted alkylaryl, arylalkyl or substituted arylalkyl; and L is a direct bond or a divalent aromatic linker containing 6-12 carbons.