In the present work, the transformation of m-xylene was studied over fresh and pre-coked H-ZSM-5 catalyst in a riser simulator over the temperature range of 350-500 degrees C. Significant differences were observed in both the activity and product selectivity pattern for the two forms of the catalyst. While, the fresh catalyst generally gave higher m-xylene conversion, the pre-coked catalyst produced higher isomerization/disproportionation (I/D) and para/ortho xylene (P/O) ratios. For both forms of the catalyst, m-xylene conversion was found to increase with both reaction temperature and reaction time (0-15 s). A maximum conversion of 30.8% was achieved at 500 degrees C for a reaction time of 15 s using the fresh catalyst. The corresponding value for the pre-coked catalyst was 24.6%. P/O and I/D ratios as high as 2.5 and 10, respectively, were observed with the pre-coked catalyst. It was also observed that the P/O and I/D ratios decreased with increasing temperature for both catalyst and that the difference between these ratios for the two catalysts was more pronounced between 400 and 450 degrees C than at higher temperatures. Kinetic modelling of the m-xylene transformation over the pre-coked catalyst yielded a lower activation energy for p-xylene formation compared to the fresh catalyst. And an activation energy of 17.5 kcal/mol for the m-xylene dispropotionation was obtained for the pre-coked catalyst compared to 7.82 kcal/mol for the fresh catalyst. These values are indicative of the restriction posed on disproportionation by the pre-coked catalyst as reflected by higher I/D ratios. (c) 2007 Elsevier B.V. All rights reserved.