The influence of the supports (Al2O3, TiO2 and SiO2) on the activity and selectivity of Pt-Ir catalysts for the ring opening (RO) of decalin was studied. It was found that the properties of the supports strongly influence the distribution, size and Pt-Ir interaction of the metal particles. They therefore also influence the catalytic activity for reactions like dehydrogenation and hydrogenolysis, but also specifically the activity for RO reactions of naphthenes. Via their acid sites the supports influence RO reactions although the effect is not very marked on conversion because the acidity differences are not very significant among them. The results show that the highest decalin conversion for three different supports, alumina, silica and titania, is obtained with the catalysts of highest metal loading. The best selectivity to RO products is obtained with Pt-Ir(1.0)/Al2O3 catalyst. Moreover this catalyst shows the highest performance for MCP RO. .