The alkylation reaction of o-xylene with styrene was studied by using various cation-exchange resin catalysts. Amber-lyst-15 catalyst was found to deactivate on repeated use. This deactivation was studied from the viewpoint of pore structural alterations. The pore structure data of the virgin and deactivated catalyst were analysed by using various techniques such as mercury porosimetry, BET surface area and SEM. The cause and mode of deactivation were investigated by carrying out series of experiments by altering different reaction parameters such as temperature, mole ratio, mode of addition of styrene, divinylbenzene content of the resin, etc. The results focus on the limitations of the ion-exchange resin catalysts, especially with reactants that can form polymeric chains with the matrix.