Alumina-supported molybdenum phosphide hydroprocessing catalysts, MoP/gamma-Al2O3, with weight loadings from 3.5 to 39 wt% were prepared by temperature-programmed reduction of alumina-supported molybdenum phosphate precursors. The precursors were obtained by incipient wetness impregnation of the support with aqueous molar solutions of ammonium paramolybdate and ammonium phosphate (Mo/P = 1/1). Effects of loading, reduction temperature, and heating rate on the catalysts were studied, and the samples were characterized by CO chemisorption, BET surface area, and X-ray diffraction (XRD) measurements. Compared to the bulk MoP and MOP/SiO2 systems (which are similar to one another), the MoP/gamma-Al2O3 material showed different behavior. Whereas calcined bulk and silica-supported molybdenum phosphates form Mo-P amorphous glasses which reduce directly to MoP, for the alumina-supported phosphates at high loadings (e.g., 13 wt% MoP/gamma-Al2O3), a MoO3 component is visible which reduces sequentially with temperature to MoO2 and Mo metal, and then transforms to MoP. The difference in behavior between these systems is attributed to the formation of a strongly bound amorphous phosphate surface layer on the alumina support. Alumina has a strong affinity for phosphates, and appears to initially abstract a substantial amount of phosphorus from the stoichiometric molybdenum phosphate mixture. The alumina then releases the phosphorus at high temperature, allowing it to recombine with Mo metal to form MoP and generating active sites. The MoP/gamma-Al2O3 catalysts performed well in hydroprocessing of a simulated distillate containing dibenzothiophene and quinoline at conditions representative of industrial hydroprocessing (643 K, 3.1 MPa). For the 13 wt% sample the hydrodesulfurization conversion was 57% and hydrodenitrogenation conversion was 62%. Catalytic activity, based on equal chemisorption sites loaded in the reactor (70 mumol), was generally independent of the amount of MoP deposited on the alumina surface, independent of the presence of X-ray visible molybdenum phases, and was associated with a relatively high temperature reduction peak found at all loading levels of MoP/Al2O3 but not in bulk MoP. We conclude that the high-temperature peak gives rise to highly dispersed MoP which is responsible for the bulk of the CO titration sites and the catalytic activity, and that the large MoP particles visible by XRD make smaller contributions. (C) 2003 Elsevier Inc. All rights reserved.