The influence of the metal function in bifunctional Pt/USY catalysts in the combined hydrogenation and ring opening of tetralin has been investigated. The key step in the reaction pathway leading to the formation of the desired ring opening products (ROPs) was the isomerization (ring contraction) of the C-6 naphthenic rings of decalins. The presence of Pt in the bifunctional Pt/USY catalyst significantly increased the rate of isomerization and hence the formation of ROP as compared to the monofunctional USY sample. The formation of the high cetane ROP increased with increasing the proximity between the Pt and acid sites and with increasing the Pt loading (i.e. the metal/acid ratio) in the range of 0.25-4 wt.%. Ring opening of naphthenes was seen to occur exclusively on the Bronsted acid sites of the zeolite. Incorporation of a hydrogenolysis metal (Ir) to the bifunctional Pt/USY catalyst did not lead to a further increase in the formation of ROP and significantly reduced the sulfur tolerance of Pt. This was ascribed to a higher electron density on the Pt sites induced by Ir in bimetallic Pt-Ir particles, as evidenced by XPS. (C) 2004 Elsevier B.V. All rights reserved.