The preparation of highly sulfonated poly(phenylene sulfone) (ion exchange capacity = 4.45 meq g(-1)) by polycondensation of sulfonated difluorodiphenyl sulfone with metal sulfide and subsequent oxidation with hydrogen peroxide is optimized. The monomer/metal sulfide ratio, polymerization time and temperature, and type of metal sulfide have been varied. For the best conditions high molecular weights of up to 300 kg mol(-1) are obtained reproducibly. This high molecular weight polyelectrolyte is meant to be used as main constituent of heterogeneous membrane structures such as interpenetrating networks, polymer blends, or simple physical mixtures with other polymers, e.g., for application in PEM fuel cells.