m-Xylene transformation has been studied on as-prepared H-Y and a series of dealuminated Y-zeolite catalysts. The conversion of m-xylene was found to increase initially with acidity, however, decreases subsequently. It was proposed that the high concentration of acid sites in the H-Y catalyst increases paring reaction, in addition to the well-known isomerization and disproportionation pathways. A significant decrease in trimethyl-benzenes (TMBs) yields was observed with both reaction temperature and catalyst acidity. The p-xylene/o-xylene (P/O) ratio was found to be independent of zeolite acidity. A higher coke deposition was found in the transformation of m-xylene over the parent H-Y as compared to the highly dealuminated USY zeolite. The formation of benzene and C-2-C-4 gases was found to be proportional to zeolite acid concentration. (C) 2004 Elsevier B.V. All rights reserved.