The transformation of ethylbenzene was carried out on intimate mixtures of 0.5 wt% Pt/Al2O3 (75 wt%) with a platinum dispersion of 100% and of HMOR zeolites (25 wt%) with Si/Al ratios between 6.6 and 180. Products result from six main transformations of ethylbenzene or/and of the reaction products: the desired isomerization of ethylbenzene into xylenes (reaction 1), disproportionation of ethylbenzene (2), dealkylation followed by hydrogenation of ethylene (3), secondary ethylbenzene-xylene transalkylation (4), hydrogenation of ethylbenzene followed by ethylcyclohexane isomerization (5), and secondary cracking of C-8 naphthenes (6). From the comparison of the product distributions over Pt/Al2O3, over HMOR and over the bifunctional catalysts, reactions 2-4 were confirmed to occur through acid catalysis and hydrogenation through metal catalysis, and reactions 1 and 6 through bifunctional catalysis. Whatever the catalyst, the selectivity to isomers increases with ethylbenzene conversion, which is due to thermodynamic limitations in reactions 2 and 5. A significant increase in selectivity to isomers is also observed when the Si/Al ratio of the mordenite component increases from 20 to 180. With these bifunctional catalysts, the initial rate of disproportionation is proportional to the square of the concentration of protonic sites, which suggests that this bimolecular reaction requires two protonic sites for its catalysis; furthermore, the rate of dealkylation. (per protonic site) depends only on the strength of the protonic sites and the change of the isomerization rate with the balance between the metallic and acidic functions is the one expected from a classical bifunctional mechanism. For the bifunctional catalysts with HMOR6.6 and -10 as acidic components, the rates of disproportionation, dealkylation, and isomerization are lower than expected and the selectivity to isomers is higher. This can be explained by strong diffusion limitations in these zeolites which, in contrast to the other samples, do not present mesopores in addition to micropores.