Fast Fourier transformation infrared spectroscopy was used to characterize the formation of bimetallic interactions of gamma-Al2O3-supported Pd-Pt catalysts. The effects of bimetallic interactions on catalyst performance were examined in long-term stability tests, carried out with a pilot plant under the same operating conditions and feed as those of a commercial diesel hydrotreating process. These tests are the hydrotreating of straight run distillate diesel at 340 degrees C, 580 psig, and H-2/oil (mol) = 2.5 over a 25 day period. The characterization results indicate that Pd precursors and pretreatment conditions affect Pd-Pt bimetallic interactions in the H-2-reduced catalysts and the catalyst performance. The Pd-Pt catalysts made from palladium(II) acetate without calcination pretreatment presented a significant Pd-Pt bimetallic interaction, as evidenced by a shift in the infrared absorption peak characteristic of terminal CO ligands adsorbed on the Pt clusters to higher energy, bridging CO on the Pd to lower energy, and a catalytic reduction of palladium(II) acetate by Pt. In contrast, no significant bimetallic interactions were observed for both catalysts made from palladium(II) acetate with a calcination at 450 degrees C in air and from palladium amine. The long-term stability tests further indicate that the Pd-Pt catalysts with bimetallic interactions have a relatively high selectivity for aromatics reduction and hydrodenitrogenation (HDN), whereas they have a relatively low selectivity for hydrodesulfurization (HDS).