The gas-phase isomerization of meta-xylene has been carried out over USY zeolite catalyst using a fluidized bed reactor under relevant reaction conditions for FCC (temperature, contact time, reactant partial pressure, catalyst/oil ratio). Experimental results clearly indicate the presence of bimolecular-isomerization pathway, in addition to the monomolecular-isomerization reaction. The disproportionation/isomerization (D/I) ratio was found to increase steadily with both m-xylene conversion and reaction temperature, whereas p-xylene/o-xylene (P/O) ratio was only mildly affected with both reaction parameters. A simplified kinetic model based on reactant-converted (RC) deactivation model was developed for the reaction and compared with the obtained experimental data. The apparent kinetic parameters were estimated using nonlinear regression analysis. (c) 2004 Elsevier B.V.. All rights reserved.