Ion-exchange resins as membranes and in granular form have been prepared by the copolymerization of divinylbenzene with various esters of p-styrene-sulphonic acid and subsequent hydrolysis. The synthesis of the sulphonated styrene monomers is described. Equilibrium data for the sodium–hydrogen ion exchange on a resin prepared in this manner are discussed in terms of postulated variations of sulphonate group environment.