The hydrodesulfurization (HDS) reactivities and the inhibition effects of H2S and NH3 were experimentally investigated for 11 difficult-to-remove sulfur compounds contained in LC-finer light gas oil using a commercial NiMo/Al2O3 hydrotreating catalyst. Among these sulfur compounds, 4-MDBT was the most reactive while 4,6-DMDBT was the least reactive. It was found that the presence of methyl groups away from the sulfur atom slightly enhanced the HDS reactivities of the methyl substituted DBTs. The observed low reactivity of 4,6-DMDBT was mainly caused by the steric hindrance of the two methyls at the 4 and 6 positions. Both H2S and NH3 significantly inhibited the HDS reaction rates for all 11 sulfur species of interest, while 4,6-DMDBT was one of the most inhibited species. At the same concentration (1.0 vol%), H2S showed a stronger inhibition effect than NH3. Measurable catalyst deactivation was observed in the course of the experiment that had a relatively even effect on the RDS of all reactants investigated in this study. Crown Copyright (C) 2004 Published by Elsevier Ltd. All rights reserved.