A series of CoNlo alumina catalysts are prepared with constant amount of Co and Mo (2.1 wt.% Co and 10 wt.% Mo) changing the amount of chelating agent nitrilotriacetic acid (NTA) from 0 to 1.2 mol NTA per mole of (Co + Mo). After impregnation, the catalysts are not calcined but directly sulfided before activity test and IR characterization. The activity for thiophene HDS is multiplied by a factor of 2.6 for the highest NTA amounts. IR analysis during sulfidation shows three important domains for decomposition of the NTA complexes: a first one between 463 and 493 K, a second sharp one around 513 K and a final one above 553 K. Adsorption of CO followed by IR spectroscopy shows that the sulfidation of Mo is almost complete at 493 K. The higher promotion level of the NTA-containing catalysts is due to the availability of the Co liberated by decomposition of the NTA complex in the region 463-493 K and above 553 K. From the very good correlation between the HDS activity and the amount of Co-promoted sites, it is concluded that NTA addition affects the concentration of the promoted sites without changing significantly their nature. (C) 2007 Elsevier B.V. All rights reserved.