Vapour phase tertiary butylation of toluene with tert-butylalcohol was studied over high silica large pore mordenite catalysts with varying silica to alumina ratios. Influence of acidity, reaction temperature, time on stream, weight hourly space velocity and concentration of the alkylating agent on the yield and selectivity of para-tert-butyl toluene was investigated. Among the isomers formed during the reaction, the para-isomer was found to be in significant amounts followed by the meta-isomer, while no ortho-isomer was found in the products as a result of steric factors. Selectivity for para-isomer reduces at higher temperatures and it goes through a maximum in the temperature range 413-433 K. Mordenite catalysts with high silica to alumina ratio, with strong acidity but lower acid site density, offer good conversion, high alkylation selectivity and better para-selectivity. The improved para-selectivity on high silica mordenites is attributed to the low isomerization activity of the product p-tert-butyltoluene on this catalyst when compared to low silica mordenites. Lower concentration of tert-butylalcohol in the feed lead to better alkylation as well as para-butylation selectivity. (C) 2004 Elsevier B.V. All rights reserved.