A preparation method for a cation exchange membrane has been proposed involving sorption of the styrene (St)-divinylbenzene (DVB) mixture into a non-porous poly vinylchloride (PVC) film. Monomer solution consisting of St, DVB and BPO (benzoylperoxide) were absorbed into a free volume of PVC. Due to the solvent effect of the monomer, PVC permitted enlarged free volume where monomers were polymerized. The St-DVB polymer chains formed semi-IPN(interpenetrating polymer networks) with PVC, making an absorbent membrane sheet. To the base membrane, cation-exchange functional groups were introduced by a general sulfonation process.