The effect of sulfur and nitrogen contents on the activity of CoMo/Al2O3 hydrotreating catalyst was investigated using model reactants such as dibenzothiophene (DBT) for hydrodesulfurization (HDS), 2,6-dimethylaniline (DMA) for hydrodenitrogenation (HDN), and 1-methylnapthalene (1-MN) for hydrodearomatization (HDA) in a batch reactor at 4 MPa and 340 degreesC. The effect of sulfur (0-2 wt.%) and nitrogen (0-0.2 wt.%) contents was studied by changing the concentrations of DBT and DMA, respectively. The presence of sulfur and nitrogen compounds together in a feed illustrated the way produced H2S and NH3 influence the catalyst properties. The sulfur content was varied at fixed nitrogen content vis-it-vis variation of nitrogen at fixed sulfur content. Interestingly, the observed relative decrease in activity at high concentrations of DBT or DMA followed similar trend. The conversion of model feed was used to find out better insight for the inhibition in hydrotreating functionalities such as C-S and C-N hydrogenolysis and hydrogenation. Among these, all catalytic functionalities were not affected at similar magnitude with added sulfur or nitrogen content. The results indicated that adsorbed S and/or N species, which caused a decrease in hydrogenolysis activities, inhibited a part of the catalytic sites. The results also indicated opposite effects of the produced H2S and NH3 on selectivity. H2S improves slightly the selectivity for hydrogenation contrary to NH3 that causes a strong decrease in hydrogenation selectivity. (C) 2004 Elsevier B.V. All rights reserved.