High silica mordenite (HSM) samples have been synthesized hydrothermally by using low aluminum contents (Si/Al molar ratios 40 and 60) with the synthesis gel in the presence of orthophosphoric acid as promoter at 443 K. Sodium silicate and aluminum sulfate were used as Si and Al precursors, respectively. Solid yield and particle size of these materials have been improved considerably over the conventional synthesis method for aluminous mordenite apart from high Si/Al molar ratios of the product and hydrothermal stability. These high silica mordenite samples showed excellent catalytic activity in the alkylation of benzene under liquid phase reaction conditions together with high selectivity towards linear alkyl benzenes. This catalytic performance suggested the presence of strong acid sites in this HSM material. (C) 2004 Elsevier B.V. All rights reserved.