The catalytic performances of unsupported nanosize MoS2- and CoMo-based Al2O3 catalysts in dibenzothiophene hydrodesulfurization have been compared. The reaction was carried out in a high-pressure batch reactor under 3 MPa of H-2. The effects of a wide range reaction temperatures along with the presence of H2S on the catalytic activity and selectivity were studied. In all catalysts, the rise in the catalytic activity with temperature was Arrhenius dependent; however, the nature of the catalysts affected the reaction selectivity. The direct fission of C-S bond is the most important route for CoMo/Al2O3 catalyst. On the other hand, the contribution from the hydrogenation reaction followed by hydrodesulfurization is most significant for the nanosize MOS2 catalyst. The results demonstrate that the coexistence of H2S in the reaction feedstock does not permanently deactivate and/or activate the catalysts. There was a severe inhibition because of H2S on the catalytic activity for the dibenzothiophene hydrodesulfurization over CoMo/Al2O3 catalyst. However, H2S causes a significant catalytic promotion for nanosize MOS2 catalyst. A drastic modification in the contribution of the reaction pathways for dibenzothiophene hydrodesulfurization in exist of H2S over the nanosize MOS2 catalyst was observed. The apparent thermodynamic parameters for the dibenzothiophene hydrodesulfurization over the present catalysts were estimated. Large variations in both the apparent activation energy and the apparent frequency factor were observed for the nanosize MOS2 catalyst investigated under different status of H2S. This reflects the differences in the energetic requirements of the hydrodesulfurization pathways in each case. The differences in the apparent activation parameters suggest that the activated complexes might differ considerably in energetic detail and that distinct activated complexes in each case might be involved in the reaction mechanism. On the contrary, there was no important change in the activation parameters due to H2S for the reaction carried out over CoMo/Al2O3 catalyst. (C) 2007 Elsevier B.V. All rights reserved.