The inhibition effects of nitrogen compounds on CoMoP/Al2O3 catalysts due to doping with Li or USY-zeolite in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) have been studied. Quinoline was selected as the model nitrogen compound. Changes in the HDS rate, conversion and selectivity were investigated. The acidity of the oxidic catalysts was studied using Fourier-transformed infrared spectroscopy (FT-IR), pyridine adsorption, and ammonia temperature-programmed desorption (NH3-TPD). The catalysts were studied in both oxidic state and sulfidic state by X-ray photoelectron spectroscopy (XPS). The HDS of DBT as a reference probe molecule is thought to proceed first through direct desulfurization pathways, then to hydrogenolysis. A significant amount of benzene was detected in the case of CoMoP/zeolite-Al2O3. The HDS of 4,6-DMDBT over CoMoP/zeolite-Al(2)03 in the absence of quinoline proceeds through a very complex mechanism that involves several other reactions, such as isomerization, alkylation, and hydrocracking. This can probably be attributed to the acidic properties of zeolite. In the presence of quinoline, a strong inhibition effect was observed for all the catalysts. The inhibition effects of quinoline on CoMoP/Al2O3-Li were smaller than its effects on the other catalysts; this fact resulted in the enhancement of HDS activity. Moreover, from the XPS investigations we can conclude that CoMoP/Al2O3-Li showed a relatively higher sulfidation degree for both Mo and Co. Thus, it seems to have a relatively higher catalytic activity. (c) 2005 Elsevier B.V All rights reserved.