The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and its hydrogenated intermediates 1,2,3,4-tetrahydro-dibenzothiophene (TH-DBT) and 1,2,3,4,4a,9b-hexahydro-dibenzothiophene (HH-DBT) over a bulk MoP catalyst was studied at 340 degrees C and 4 MPa both in the presence and absence of piperidine. The results indicated that sulfur was incorporated on the surface of MoP during HDS reactions, probably leading to the formation of new active sites, which possessed higher direct desulfurization (DDS) activity than the fresh MoP. The hydrogenation pathway and DDS pathway played an equally important role in the HDS of DBT. The desulfurization of TH-DBT was much faster than that of DBT, whereas HH-DBT mainly desulfurized by dehydrogenation to TH-DBT and subsequent desulfurization of TH-DBT. Piperidine decreased the rates of all reactions, but that of hydrogenation more than of desulfurization. It not only competed with the sulfur-containing molecules for adsorption on the active sites but also slowed down the sulfidation of MoP surface. (C) 2011 Elsevier Inc. All rights reserved.