Nanosized MoS2 catalysts with basal planes of similar to 4 nm were synthesized by thermally annealing ammonium tetrathiomolybdate. The structure and morphology of the obtained phases were determined by XRD, SEM and TEM. These nanosized MoS2 clusters have a truncated hexagonal shape. The MoS2 layers in these batches were found to be curved to some extent. Catalytic performance was evaluated by the hydrodesulfurization of dibenzothiophene. We studied the effect of a wide range of H2S partial pressures on the hydrodesulfurization activity and selectivity. Catalytic activity for the hydrodesulfurization of dibenzothiophene increased remarkably when H2S was included within the reaction zone. The role of the curved MoS2 layers on activity is discussed. (C) 2009 Elsevier B.V. All rights reserved.