Alkylation of in-cresol with cyclopentene in presence of benzenesulphonic acid was studied statistically with a three-factored experimental design. Factorial design was employed to study the effects of single factors and the effects of their interactions on the yields of alkylation. Reaction temperature, molar ratio of m-cresol to cyclopentene and amount of benzenesulphonic acid were considered as the major variables. A mathematical model was derived to calculate the predicted yield of alkylation of m-cresol with cyclopentene in presence of benzenesulphonic acid under certain reaction conditions. The adequacy of the suggested model was checked up. An optimum yield (about 94%) of the product was obtained under the reaction conditions of a temperature of 140 degrees C; a 5:1 molar ratio of m-cresol to cyclopentene and a 8% by weight benzenesulphonic acid of m-cresol. (C) 2007 Elsevier B.V. All rights reserved.