CoMoS catalysts with constant Co and Mo contents were prepared with different amounts of nitrilo triacetic acid (NTA) chelating agent. NTA has a marked positive effect on the catalyst activity for gas-phase reactions such as atmospheric pressure thiophene hydrodesulfurization (HDS) and high-pressure 2,6-dimethyl aniline hydrodenitrogenation, whereas no effect is found for liquid-phase dibenzothiophene HDS. Transmission electron microscopy analysis shows that NTA has no effect on MoS2 slab size and stacking, while infrared spectroscopy of adsorbed CO reveals a strong increase in the amount of Co-promoted sites. Comparison of activity and spectroscopic data implies that increase of Co-promoted sites accounts for the catalytic activity enhancement, but it also provides evidences for the creation of different kinds of sites on the NTA catalysts. Various possibilities (type II CoMoS phase, Co clusters, or CoMoSN) are discussed. (C) 2009 Elsevier Inc. All rights reserved.