The gas-phase transformation of mixtures of benzene and xylenes has been studied on several zeolites. Transalkylation between benzene and xylenes and disproportionation and isomerization of xylenes are the main reactions in this process. Under the reaction conditions used, medium pore zeolites (FER and ZS M-5) were inactive, whereas large pore zeolites (USY, BETA and MOR) yielded toluene and trimethyl benzenes on a different extension. The catalytic activity has been related to the acid properties and so MOR(20) exhibited the highest overall conversion. The relative contributions to the transalkylation and disproportionation reactions were found to be influenced by the feed composition (i.e. the benzene to xylene ratic), the strength of the acid sites and the pore size and geometry of the catalysts. All zeolites underwent a substantial deactivation by coke deposition, particularly when o-xylene was used as reactant. Pt-supported zeolites were used in order to improve the stability of the catalysts. (C) 2004 Elsevier B.V. All rights reserved.