Heterocyclic nitrogen compounds in crude oil, such as pyridine and quinoline, are known to have a strongly disabling effect on the hydrodesulfurization (HDS) reactions on MoS2-based catalysts. To shed light on the nature and distribution of the adsorption sites of such nitrogen-containing inhibitors, we use atom-resolved scanning tunneling microscopy (STM) on a gold-supported HDS model system to probe the detailed adsorption configurations and diffusion characteristics of pyridine and quinoline molecules on both unpromoted and Co-promoted MoS2 (CoMoS) nanoparticles. We find that pyridine and quinoline molecules adsorb strongly on the S-edges of reduced MoS2 and CoMoS nanoparticles through the formation of protonated pyridinium/quinolinium ions. The quinoline molecules adsorbed in a flat configuration on the S-edge are concluded to block significantly more active sites than the pyridine molecules, due to the larger molecular size and higher mobility. The experimental observations also illustrate how proton transfer to the adsorbed N-containing species may affects bonding of the terminal S atoms of the active edge to the Au substrate. The STM findings overall substantiate the notion that these N-bearing compounds compete with the adsorption of sulfur-containing molecules on the same active edge sites in hydrogenation reactions, but also indicate that corner sites may be less affected. (C) 2018 Published by Elsevier Inc.