Two unpromoted Mo/gamma-Al2O3 catalysts containing almost the same amount of Mo were prepared using equilibrium deposition filtration, EDF, for depositing Mo-oxo species on the alumina surface at two different pH values (3.9 and 6.3). On these samples the same amount of Co ions was deposited by dry impregnation and thus, the corresponding CoMo/gamma-Al2O3 catalysts were prepared. The unpromoted and the Co-promoted samples were characterized using BET, DRS, LRS, TPR, and NO chemisorption. Moreover, the catalytic activity of the samples was determined at various temperatures using the hydrodesulfurization of thiophene as a probe reaction. The characterization of the specimens showed that the Mo deposition at pH = 3.9 favors the deposition of polymeric Mo-oxo species, in agreement with predictions drawn on the basis of two models related with the acid-base behavior of the gamma-Al2O3 surface. These species after calcination provide a well-dispersed Mo phase with relatively high reducibility. This phase, in which the concentration of the octahedral Mo is greater compared to that on the Mo/gamma-Al2O3 specimen prepared at pH = 6.3, hinders, in turn, the formation of the catalytically inactive CoAl2O4, whereas after sulfidation this provides a sulfided state with relatively high concentration of sulfur vacancies. The above explain why the unpromoted and the Co-promoted catalysts prepared at pH = 3.9 proved to be more active than the corresponding catalysts prepared at pH = 6.3.