Bulk molybdenum sulfide catalyst was prepared by thermal decomposition of ammonium heptamolybdate at 800 degreesC in a flow of a 10% (v/v) H2S/H-2 gas mixture. Investigation of the catalytic activity of the grinding catalyst for hydrodesulfurization (HDS) of dibenzothiophene (DBT) was carried out in a batch magnetically stirring microautoclave reactor at 340 degreesC and 3 MPa H-2. The influence of a wide range of H2S concentration on the catalytic activity was studied. A significant promoting effect on HDS of DBT by H2S was observed. The experimental results showed the great enhanced contribution from the hydrogenation route with H2S in HDS of DBT. However, severe inhibition by H2S against the direct desulfurization route was observed. Kinetic analysis leads to the individual reaction rate constant in the reaction scheme performed.