The effect of Pt/(Pt + Mo) atomic ratio, metal content and the characteristics of the alumina support on activity and synergy in hydrotreatment (HDT) reactions were studied. Pt-Mo supported on different commercial gamma alumina (gamma-Al2O3) catalysts with different metal contents were prepared and evaluated in simultaneous dibenzothiophene (DBT) hydrodesulfurization (HDS) and naphthalene hydrogenation (HYD) reactions. The characterization of the catalysts was performed using CO chemisorption, proton affinity distribution (PAD) and infrared (IR) spectroscopy of adsorbed NO. The Pt/(Pt + Mo) atomic ratio drastically influences the catalytic behavior. The formation of active sites with high activity and an important synergistic effect is enhanced at a very specific low metals composition of the catalysts. The characterization results suggest that the enhanced HDS and HYD activities are due to an improved dispersion of the active species on the support, which was reached at low metal contents and a specific atomic ratio. Moreover, this improved dispersion was influenced by the surface composition of the alumina and could be associated with a specific interaction between the Pt and the hydroxyl groups of the alumina as well as with the formation and distribution of the Mo active phase exposed on the support. (c) 2006 Elsevier B.V. All rights reserved.