From an interplay of Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) experiments, and density functional theory (DFT) calculations, we investigate the fundamental effect of the use of organosulfur compounds for the sulfidation of MoS2 nanoclusters in the hydrotreating catalyst. Our results reveal that incorporation of carbon in MoS2-based hydrotreating catalysts as carbidetype phase is not favorable when synthesized with or exposed to dimethyl disulfide (DMDS) or dimethyl sulfide (DMS). These results suggest that substitution of sulfur with carbon on the predominant type of edge structures in MoS2 nanoclusters is thermodynamically unfavorable, a result that is confirmed by DFT to be valid for all edge structures of MoS2 exposed under catalytically relevant conditions. However, the results show that the choice of the sulfiding agent can strongly influence the morphology and dispersion of the sulfided phase, and such effects are therefore expected to be of major importance for the activity of the freshly sulfided catalyst. (C) 2011 Elsevier Inc. All rights reserved.