The influence on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) of different activation procedures using H2S or dimethyldisulfide (DMDS) has been investigated on phosphorus-free and phosphorus-doped CoMo and NiMo industrial catalysts supported on alumina. The comparison of the two sulfiding agents was performed using strictly similar procedures of sulfidation to determine the origin of the well-known beneficial effect of organosulfides for the activation of hydrotreating catalysts. Partial pressures in H-2 and/or in sulfiding agents have been modified to observe activity variations among the different sulfiding agents used. Comparison between H2S and DMDS has also been performed on phosphorus-doped CoMoP/Al2O3 and NiMoP/Al2O3 catalysts. Results emphasized the importance of supplying H2S to the catalyst at low temperatures of activation (around 423 K) even in a low proportion (P-H2S as low as 3.3 kPa) to sulfide correctly NiMo and CoMo catalysts. Indeed, through a low consumption of H2S, O-S exchange and intramolecular redox reactions can be initiated at low temperatures of activation leading to a level of sulfidation sufficiently advanced before reaching temperatures (above 573 K) at which a strong competition between reduction and sulfidation starts. DMDS appears less efficient than H2S for activating NiMo and CoMo catalyst due to its inability to provide H2S to the catalyst at these low temperatures of activation. At higher temperatures (T > 543 K), a good activation cannot be obtained if a high P-H2/P-H2S ratio is used whatever the sulfiding agent. Finally, the difficulty of activating the P-doped CoMoP/Al2O3 and NiMoP/Al2O3 catalysts was confirmed. This study confirms that the beneficial role of organosulfide compounds as activating agents is not related to a pure "chemical" phenomenon but more probably to a "thermal well" effect limiting the exothermic character of the oxide-sulfide transformation. (c) 2005 Elsevier B.V. All rights reserved.