A series of bimetallic Pd-Pt (mole ratio Pd : Pt = 4 : 1) catalysts supported on HY and USY (ultrastable Y) zeolites for a wide range of SiO2/Al2O3 ratio (5.6-680) was prepared and characterized by chemical analysis, XRD, N-2 adsorption, IR spectroscopy of pyridine adsorption, and CO adsorption. The hydrogenation of tetralin was done over the Pd-Pt/HY and Pd-Pt/USY catalysts at a hydrogen pressure of 3.9 MPa at 553 K in the presence of dibenzothiophene at a sulfur concentration of 500 ppm. The hydrogenation activity, selectivity, and the sulfur tolerance strongly depended on the SiO2/Al2O3 ratio of the zeolites. The activity increased with increasing SiO2/Al2O3 ratio, peaked when the ratio was in the 15.0-40 range, and then decreased as the ratio increased further. The IR analysis showed that for the USY zeolites with a SiO2/Al2O3 ratio of 15.0 or above, the amount of Bronsted acid sites was extremely small, while strong Lewis acid sites existed on the surface of these zeolites. The decrease in both the activity and the sulfur tolerance of Pd-Pt/USY catalysts when the SiO2/Al2O3 ratio increases from 15.0-40 to 680 may be primarily due to the decrease in the amount of electron-deficient Pd-Pt resulting from the decrease in Lewis acidity. Compared with the Pd-Pt/USY (SiO2/Al2O3 = 15.0-40) catalysts, the activities of the Pd-Pt/HY and Pd-Pt/USY (SiO2/Al2O3 = 10.7) catalysts were lower, probably because of pore diffusional limitations.