The alkylation and disproportionation of ethylbenzene (EB) in the presence of aromatics like m- and p-xylene isomers have been studied over a pore-size regulated HZSM-5 catalyst. The industrial feed having different compositions of ethylbenzene and xylene isomers were used for the experimentation. Hence, they were expected to hinder the movement of reactant molecules both on the external surface and within zeolite channels. It was observed that irrespective of the different feed compositions the concentration of the xylene isomers were intact in the product. There is no other byproducts formation like p-ethylmethyl benzene. The effect of varying the concentration of aromatic compounds in the feed on ethylbenzene conversion and product distribution over the parent and modified HZSM-5 catalyst have been discussed. The effect of catalyst LID (length of the catalyst bed/inner diameter of the reactor) ratio across the reactor on the ethylbenzene conversion and selective formation of p-diethylbenzene (p-DEB) is also discussed. (C) 2004 Elsevier B.V. All rights reserved.