Two series of (Co)Mo/Al(2)O(3) catalysts were compared: one set was prepared with a constant amount of Mo, and various amounts of Co (from 0 to 4.5%), the second one containing the same amount of metals, but with ethylene diamino tetraacetic acid (EDTA) addition (27 wt%). The sulfided catalysts were characterized by IR spectroscopy of adsorbed CO. IR reveals that EDTA leads to a strong increase in the concentration of Co-promoted sites. EDTA has also a significant positive effect on thiophene hydrodesulfurization at atmospheric pressure, and 2,6-dimethylaniline hydrodenitrogenation at high pressure. Combination of spectroscopic and activity data shows that EDTA chiefly increases the amount of promoted sites having lower average intrinsic activities. This decrease in intrinsic activity should be due to a change of the site structure. Two models can be proposed to account for EDTA effect on the site structure such as the formation of type II slabs or that of nitrogeno-sulfide Co-promoted sites, “CoMoSN”. (C) 2009 Elsevier B.V. All rights reserved.