Hydrogenation of naphthalene and tetralin in the presence of CO was studied over alumina-supported Co, Ni, Ru, Rh, Pd and Pt catalysts. In the presence of CO, naphthalene was hydrogenated selectively without CO hydrogenation over Pd/Al2O3, Whereas both naphthalene and CO were hydrogenated over Co/Al2O3, Ni/Al2O3, Ru/Al2O3 and Rh/Al2O3. The relative activity of six supported metals was dependent on the presence or absence of CO. Rh/Al2O3 showed very low activity for hydrogenation of naphthalene and tetralin with 2%CO-H-2, while it showed the highest activity for hydrogenation with pure hydrogen. In contrast, Pd/Al2O3, showed the highest activity for naphthalene and tetralin hydrogenation in the presence of CO, From FTIR measurements of adsorbed CO, the high activity for naphthalene and tetralin hydrogenation over Pd/Al2O3 in the presence of CO Was correlated with desorption of CO (>ca. 1950 cm(-1)) adsorbed on coordinatively unsaturated sites.