HDS activity of 4,6-DMDBT for calcined and non-calcined CoMoP/Al2O3 catalysts prepared with Co7/2PMo12O40 was compared with that of a catalyst prepared conventionally by coimpregnation with a solution containing ammonium heptamolybdate, cobalt nitrate, and phosphoric acid. The extent of promotion for each method of preparation was quantitatively determined by CO adsorption, and the intrinsic activity of the different coordinatively unsaturated Mo sites in the catalyst was evaluated. The use of Co7/2PMo12O40 as catalyst precursor leads to higher HDS activity than the catalyst prepared by coimpregnation. The high activity of the former catalyst was related to a higher number of Co-promoted molybdenum sites. The number of Co-promoted sites depends on the pretreatment before sulfidation, particularly calcination. Highest activity is obtained by avoiding calcination, and the activity decreases with the calcination temperature. Two different types of promoted CoMoS sites with different HDS intrinsic activities seem to coexist in the sulfided catalyst. (C) 2011 Elsevier Inc. All rights reserved.