The present work focuses on the effect of nitrogen compounds on the activity of MCM-41- and gamma-Al2O3 -supported Co-Mo catalysts for the deep hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a fixed-bed flow reactor. Sulfur removal to the depths required by new specifications will require knowledge of the influence of non-sulfur diesel fuel components on deep hydrodesuffurization. The main objective of this paper is to examine the activity of hydrodesulfurization catalysts during and, most importantly, after exposure to basic and non-basic nitrogen. Quinoline (basic nitrogen) inhibits catalytic activity of both gamma-Al2O3- and MCM-41-supported catalysts. It strongly inhibits hydrogenation and hydrogenolysis activity as evidenced by decreased selectivity for cyclohexylbenzene and biphenyl derivatives, respectively. To a certain extent, the long-term effects of quinoline are reversible. Carbazole (non-basic nitrogen) has little effect on the gamma-Al2O3 -supported Co-Mo catalyst but significantly inhibits the activity of the MCM-41-supported Co-Mo catalyst. The inhibition of the MCM-41-supported catalyst is reversible following removal of carbazole from the feedstock. Molecular modeling was also conducted to derive the bond order and electron charges of the nitrogen and sulfur compounds, which are helpful to understanding the experimental results. (C) 2003 Elsevier B.V. All rights reserved.