Two intermediates of dibenzothiophene (DBT)-tetrahydro-DBT (THDBT) and hexahydro-DBT (HHDBT)-were synthesized, and their hydrodesulfurization (HDS) mechanism was investigated over Mo/gamma-Al2O3 at 300-340 degrees C and 5 MPa in the absence and presence of H2S and 2-methylpiperidine. The rate constants of all steps in the kinetic network of the HDS of DBT were measured. THDBT underwent desulfurization by hydrogenolysis to 1-phenylcyclohexene, followed by hydrogenation to pherylcyclohexane. The desulfurization of HHDBT occurred by hydrogenolysis of the aryl C-S bond and then cleavage of the cycloalkyl C-S bond of the resulting thiol by elimination to I-phenylcyclohexene and by hydrogenolysis to phenylcycloliexane. H2S strongly inhibited the desulfurization of all three molecules but did not inhibit (de)hydrogenation. 2-Methylpiperidine also had a strong inhibitory effect, especially on (de)hydrogenation and, to a lesser extent, on desulfurization. The order of the inhibition of DBT, THDBT, and HHDBT was explained by the adsorption constants of these three molecules. (C) 2008 Elsevier Inc. All rights reserved.