We examined effects of crosslinking agents, i.e., ethyleneglycol dimethacrylate (EGD) and butanediol dimethacrylate (BDD) containing ester groups on chloromethylation of crosslinked polystyrene resin matrices. It was proved that the ester group in methylmethacrylate (MMA) accelerates the chloromethylation of the divinylbenzene (DVB)-crosslinked styrene-MMA copolymer. As the MMA content increased in the styrene-MMA copolymers, the chloromethylation was enhanced. Complete chloromethylation was obtained at about 25% MMA content.