The nature of the active sites on the NiMoS catalyst was explored using the combination of the atoms in molecules theory and an electrostatic potential based methodology. Direct visualization of the basin of the outermost atoms of the studied NiMoS edge models has shown that the Mo atoms are practically covered by the outermost S atoms while the promoter Ni are the most accessible atoms to the exterior of the edges. Electrostatic potential mapping on the atomic border exposed to the exterior of the edge and adsorption of pyridine as the probe molecule have shown that the adsorption of a molecule containing the pollutant atoms strongly depends on two factors: the Lewis acidity of the site and the repulsive interaction of the incoming molecule with sulfur atoms impeding the free access to the site. In this sense, the main role of the promoter is to produce sulfur uncovered Lewis acid sites. Extraction of the outermost S atom creates a very strong Lewis acid site locate at uncovered Mo atoms. (C) 2007 Elsevier B.V. All rights reserved.