A systematic study on the influence of reaction conditions (temperature, time, and reactant type) on the selectivity of xylene transformation has been carried out over USY zeolite. Initial product selectivity revealed that both isomerization and disproportionation of xylenes are primary reactions. Higher conversion was observed with p-xylene reactant as compared to m- and o-xylene. The "reactant converted" deactivation model was tested with xylenes as model compounds, using effective, mechanistic kinetic models, developed from the transformation of each xylene isomers. The parameters optimized to the experimental data gave a good prediction of the overall reaction kinetics for the xylene transformations. The results provide significant evidence that the "reactant converted" decay model can be adequately utilized to account for catalyst deactivation of the xylene transformation. (C) 2004 Elsevier B.V. All rights reserved.