A series of Ir-Pd/SiO2-Al2O3 catalysts has been prepared by incipient wetness co-impregnation of acetylacetonate precursors and treated by reductive thermal decomposition. The decomposition has been followed by combined thermogravimetry, differential thermal analysis, and mass spectrometry, showing a marked difference between the bimetallic and monometallic cases. The catalysts characterization by transmission electron microscopy and single-nanoparticle energy dispersive X-ray spectroscopy reveals that the particles are bimetallic and exhibit a size-composition correlation. More generally, the enrichment of larger particles with the less cohesive element (Pd in the present case) via Ostwald ripening is here claimed to be inherent to the thermal activation treatments used in the preparation of bimetallic catalysts. The catalysts have been tested in the hydroconversion of tetralin, which is considered as a model molecule for gas oil upgrading by selective ring opening. At 350 degrees C, 4 MPa H-2, and in the presence of ppm amounts of H2S, hydrogenation, dehydrogenation, and ring-opening/contraction products (ROCPs) are formed. While the activity increases with the Pd content, the selectivity to ROCPs reaches a maximum for the Ir55-Pd45 composition and increases with the sulfur concentration. The structural and catalytic results are combined to propose a qualitative model relying on metal-acid bifunctionality and size-composition correlation. While the particle size drives the number of surface sites, the corresponding surface composition governs the hydrogenation efficiency of these sites. Therefore, by controlling the mean metal composition of the catalyst, it is possible to tune the activity-selectivity balance. (C) 2012 Elsevier Inc. All rights reserved.