The inhibiting effect of nitrogen compounds on the hydrogenation of aromatic compounds and on the hydrodesulphurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene (4,6 DMDBT), has been systematically investigated over a commercial NiMo hydrotreating catalyst. Amongst the different nitrogen compounds considered, interestingly ammonia was found to have the strongest inhibition effect on hydrogenation as well as on hydrodesulfurization reactions. The inhibiting effects were found to increase in the order quinoline < tetrahydroquinoline < indole < indoline < ammonia for phenanthrene/tetralin hydrogenation and HDS reactions. The molecules with nitrogen atoms having higher negative Mulliken charges showed higher adsorption constants.