Supported Pt catalysts were modified by adding a second metal such as Co, Mo, Ni, Re, Ag, and Pd. The effects of a second metal on the sulfur resistance were first investigated by accelerated aging test reactions. The test reactions were hydrogenation of tetralin containing 1000 ppm sulfur at 280 degrees C, 380 psig, and H-2/oil (mel) = 2.7. The results indicated that Pd-Pt catalyst had the highest sulfur resistance. Aromatics reduction, hydrodesulfurization, and hydrodenitrogenation of straight run distillate diesel over supported Pt and Pd-Pt catalysts were further examined by long-term stability tests. The operation conditions of these tests are the same as those run in a commercial diesel hydrotreating process. The results again indicated that the Pd-Pt catalyst exhibited much better catalytic performance. These results, together with electronic properties of the metal clusters on the catalysts characterized by FFT-IR spectroscopy, provide a basis to speculate the role of Pd in enhancing sulfur resistance; the Pd-Pt bimetallic interactions decrease the electron density of Pt and thereby inhibit the adsorption of H2S.