The effect of phosphorus and nickel on alumina-supported molybdenum oxycarbides has been studied. Catalysts were characterized by elemental analysis, X-ray diffraction, CO chemisorption, transmission electron microscopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. Propene and tetralin hydrogenation was used to probe the activity of the doped materials. Hydrodesulfurization of 4,6-dimethyidibenzothiophene (613 K and 4 MPa total pressure) was investigated as a probe for the deep HDS of diesel fuel. For contact times ranging from 0 to 0.4 s (conversion < 45%), the HDS of 4,6-DMDBT is zero order with respect to the reactant and proceeds via two parallel routes, direct desulfurization and hydrogenation pathways, the former being preponderant. Phosphorus modifies catalytic behavior in several ways: increase in the Mo4+ abundance, increase in the number of Lewis acid sites, and consequent enhancement of the DDS rate. Nickel induces the highest HDS activity; a nickel-molybdenum interaction observed by FTIR spectroscopy of adsorbed CO could be responsible for this behavior. (C) 2003 Elsevier Inc. All rights reserved.