Novel proton conducting ionomers have been prepared by attaching pendant sulfonated aromatic side chains to polysulfone. Lithiated polysulfone was first reacted with 4-fluorobenzoic acid chloride to introduce 4-fluorobenzoyl side chains to the polymer main chain. Next, the activated fluoro groups were replaced by 4-sulfophenoxy or 7-sulfo-2-naphthoxy in a potassium carbonate-mediated nucleophilic substitution reaction. This reaction proceeded under full conversion and the degree of substitution was easily controlled by the degree of lithiation in the first step. Membranes based on ionomers carrying one sulfophenoxybenzoyl unit per polymer repeat unit reached a proton conductivity exceeding 30 mS center dot cm(-1) at 120 degrees C under immersed conditions.