Catalytic transformation of 1,3,5-trimethylbenzene (TMB) has been investigated over a USY zeolite catalyst in a novel riser simulator at different operating conditions. The effect of reaction conditions on the variation of isomerization to disproportionation products ratio, distribution of TMB isomers, xylene/tetramethylbenzenes ratio, and values of p-xylene/o-xylene ratios are reported. Comparisons are made between the results of the 1,3,5-TMB and the earlier reported values of 1,2,4-TMB under similar conditions. Diffusion limitations were observed for the 1,3,5-TMB transformation reactions, while very mild transport effects were seen for the 1,2,4-TMB molecule. This is as a consequence of the kinetic diameters of the reacting molecules (8.6 angstrom for 1,3,5-TMB and 7.6 angstrom for 1,2,4-TMB), which are nearly the size of the passageway of the USY zeolite (7.4 A). Surprisingly, 1,3,5-TMB was found to be more reactive than 1,2,4-TMB at temperatures above 450 degrees C and reaction times more than 5 s. The experimental results have been modeled using quasi-steady state approximations. Kinetic parameters for 1,2,4- and 1,3,5-TMB conversions into isomerization and disproportionation products were calculated using the catalyst activity decay function based on time on stream. The pparent activation energies were found to decrease as follows: E-1,E-3,E-5-TMB,E-disproportionation > E(1,3,5-TMB,)isomerization approximate to E-1,E-2,E-4-TMB,E-isomerization > E-1,E-2,E-4-TMB,E-disproportionation.