The adsorption of quinoline, acridine, indole, and carbazole on the well-defined NiMoS hydrotreating catalyst edge surface has been studied by means of density-functional theory (DFT) using a periodic supercell model. Quinoline and acridine, the basic nitrogen-containing molecules present in heavy oils, are preferably adsorbed on the Ni-edge surface through the lone pair electrons of the nitrogen atom, which produces relatively high adsorption energies (-Delta E-a = 16-26 kcal mol(-1)). Indole and carbazole, the non-basic nitrogen-containing molecules, primarily interact with the NiMoS catalyst edge surface through the pi-electrons of the carbon atoms. While indole preferentially adsorbs on the NiMoS surface through the P-carbon of the pyrrolic ring (-Delta E-a = 19 kcal mol(-1)), carbazole primarily interacts with the NiMoS surface through the phenyl rings (-Delta E-a = 13 kcal mol(-1)). The relative adsorptivities and energetically preferred adsorption modes of the nitrogen-containing molecules in heavy oils can provide insights into experimental observations about hydrodenitrogenation (HDN) kinetics and reaction pathways. (c) 2005 Elsevier B.V. All rights reserved.