The hydrodesulfurization (HDS) of a diesel oil was carried out in a batch autoclave reactor over the temperature range 280-420-degrees-C for 0-90 min under a total pressure of 2.9 MPa, using CoMo and NiMo catalysts in both one and two stages. The HDS reactivities of benzothiophenes, dibenzothiophenes (DBTs), and their alkylated homologes existing in the diesel fuel were examined in detail by means of respective quantitative analyses. The sulfur compounds can be classified into four groups according to their HDS reactivities which were described by their pseudo-first-order rate constants. DBTs carrying two alkyl substituents at the 4- and 6-positions, respectively, were the most resistant to desulfurization. H2S produced from reactive sulfur compounds in the early stage of the reaction is one of the main inhibitors for HDS of the unreactive species. A second stage using fresh hydrogen solved this inhibition problem, with NiMo achieving deeper desulfurization.