The effect of water extraction on the surface structure and gas oil HDS activity of alumina-supported Mo and NiMo catalysts, prepared at acidic and near neutral pH, and by sequential and simultaneous impregnation procedures, was studied by measuring structural characterization using temperature-programmed reduction (TPR) and catalytic activity before and after metal extraction with water From oxide catalysts, depending on their preparation. significant amounts of Mo and Ni present as heteropolycompounds, and crystalline MoO3 were extracted with water. The increase in catalyst activity observed after water extraction indicates that the water-soluble heteropolymolybdate species are not the oxide precursors of the active sulfide phase, and that a portion of Mo does not contribute significantly to the catalytic activity due to its poor dispersion. This effect was more pronounced on the catalysts prepared at acidic pH, where the formation of bulk MoO3 was observed, therefore, rendering, the catalysts less active. No clear correlation was obtained between HDS activity and TPR data for oxide catalysts. However, on sulfided catalysts, a satisfactory Linear correlation was found between the H-2 consumption of the reduction peak observed in the low-temperature region 390-523 K and the HDS activity of water-extracted NiMo catalysts, where Mo species are more homogeneously dispersed on the support. The nonextracted NiMo catalysts were, however, outside of this con elation due to the higher heterogeneity of the Mo species. The low-temperature reduction peak (390-523 K) of sulfided catalysts is associated with the hydrogenation of nonstoichiometric sulfur species and the adsorption of hydrogen on anionic vacancies.