Following the numerous and contradictory results reported in the literature about the origin of the different functionalities of MoS2 based catalysts, an approach using the 4,6-dimethyldibenzothiophene as a probe molecule is reported. That has been done by the study of the variations of selectivities during poisoning of 4,6-DMDBT HDS (using olefin, aromatics and nitrogen containing compounds), and by a careful study of the variations of selectivities during the transformation of 4,6-DMDBT over fresh and aged catalysts. The ratio of selectivities between the hydrogenation (HYD) and the direct desulfurization (DDS) routes are found to be the same whatever the inhibiting compound and the aging has the same effect on both routes of reaction. These results will be discussed assuming adsorptions on the same type of sites (uncoordinated molybdenum atom) and a general mechanism will be proposed. (c) 2005 Elsevier B.V. All rights reserved.