Sterical hindrance of large S-containing molecules is known to be a significant cause of low hydrodesulfurization activity. To shed light on the atomistic aspects of such steric effects, we use atom-resolved scanning tunneling microscopy (STM) to probe the detailed adsorption configurations of dibenzothiophene (DBT) and alkyl-substituted DBT molecules on both unpromoted and Co-promoted MoS2 nanoclusters. We show that the unsubstituted DBT molecule can adsorb directly through its sulfur atom at sulfur vacancies on the nanocluster. However, only vacancies at corner sites are found to be accessible for DBT adsorption. In a similar way, DBT is found to adsorb on the Co-Mo-S sites exposed at the corners of the Co-promoted nanoclusters. In contrast, the 4,6-DMDBT molecule adsorbs in a flat configuration on the brim sites of the MoS2 clusters without the requirement of a nearby vacancy. Our findings support a model where hydrogenation and sulfur extrusion may take place on different sites. (C) 2012 Elsevier Inc. All rights reserved.