A series of co-impregnated PNiMo/Al2O3 catalysts with phosphorus concentration varying from 0 to 13.6 wt% P2O5 was studied by temperature programmed reduction (TPR) in their oxidic state before and after water extraction. Diffuse reflectance (DRS) and infrared spectra (LR) of the samples after the water extraction procedure were also analyzed. The TPR data showed that the introduction of phosphorus in the co-impregnated PNiMo/Al2O3 catalysts increases the formation of easily reducible polymeric molybdenum structures at low phosphorus concentrations and of Ni-Mo microcrystallites in the samples with high phosphorus content. It was found that water extraction procedure affects molybdenum in easily reducible forms, as polymeric molybdenum and nickel-molybdenum structures, and Ni2+(Oh) ions incorporated in them. Indications were found that the increased concentration of phosphorus in the PNiMo/Al2O3 catalysts induces the formation and the stabilization of a beta-NiMoO4 phase, which has higher reduction temperature and lower dispersion than polymolybdate surface species and therefore, its formation leads to a decrease in the hydrodesulfurization activity of the catalysts with high phosphorus content.