The effects of dibenzothiophene or quinoline poisoning on the activity and selectivity of Y-zeolite-supported Pt-Pd catalysts, in the hydrogenation of tetralin at 523 K and 70 bar initial pressure, were studied in a high pressure batch reactor as a function of composition. It was found that both poisons influenced, in very similar ways, the patterns of tolerance to poisons and of cis/trans selectivity vs. catalyst composition.