A series of MoO3/Al2O3 catalysts of varying weight loading were characterized after calcination by their buffering capacity during potentiometric titration. Strong buffering of the aqueous electrolyte occurred in distinct pH ranges, which indicated the formation of hydrolysis products of the surface compounds formed in these pH windows. Addition of cobalt, followed by calcination, revealed a new feature which signaled the formation of a possible surface heteropolymolybdate compound. If cobalt was added first this compound was not formed, however, if cobalt and molybdenum were coimpregnated it was detectable. As a promoter, Ni(II) had an effect similar to cobalt, but Fe(III) did not. The series of catalysts were further tested for their hydrodesulfurization activity using thiophene as a reactant. The thiophene conversion, as a function of the amount of surface compound formed by addition of the cobalt promoter, resulted in a linear relation, indicating that the Co/Mo compound detected in the oxidic state could be correlated with the HDS activity of these catalysts.