Infrared spectroscopy was used to characterize mixed ruthenium-nickel sulphides prepared by step-wise impregnation over alumina and further sulphidation. The hydrogenation of biphenyl was compared over samples containing different amounts of nickel and ruthenium and a large increase was observed for a catalyst with an atomic Ni /(Ni + Ru) ratio near 0.4. Infrared spectra of adsorbed CO indicate that the increase in catalytic activity is ascribed to the presence of a very large number of Ru-Ni sites.