Ethylbenzene transformation was carried out on bifunctional catalysts constituted by Pt/Al2O3 (PtA) associated with NaHMOR (Si/Al = 10) zeolite samples. The operating conditions were the following: fixed bed reactor, temperature 683 K, pressure of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity (WHSV) between 5 and 150 h(-1). Twelve different bifunctional catalysts were used differing by the Na exchange rate of the mordenite component or by the relative proportions of the NaHMOR and Pt/Al2O3 components or by the Pt content of the Pt/Al2O3 (0.5 or 2.3 wt.%). As expected from a bifunctional mechanism, the activity in isomerization per protonic site (turnover frequency, TOF) increases first with the ratio between the concentrations of accessible platinum sites (n(Pt)) and of the protonic sites (n(H)+), then remains constant above a certain value of this ratio. It is shown that the exchange of the mordenite causes a significant increase in the selectivity to isomers of the bifunctional catalysts. This increase can be related to a decrease in the rates of the main secondary reaction (disproportionation), but also of the other ones: dealkylation, hydrocracking, trans alkylation.