Chemical and physical studies of MoS2 single crystals, microcrystals, and powders are carried out with the goal of understanding the relation between the morphology of the crystallites and their catalytic properties. Chemical etching of microcrystals is used to create new catalytic sites, which are responsible for the DBT (dibenzothiophene) hydrogenation reaction. Average stacking heights of poorly crystalline MoS2 powders are then estimated by X-ray crystallography and for the first time correlated with the selectivity of the catalyst for hydrogenation versus HDS (hydrodesulfurization). This result is interpreted in terms of a simple physical model called the "rim-edge" model, where the catalyst particle can be described as a stack of several discs. The top and bottom discs are associated with the rim sites. The discs "sandwiched" between the top and bottom discs are associated with the edge sites. Sulfur hydrogenolysis is obtained on both the rim and edge sites, whereas the DBT hydrogenation occurs exclusively on the rim sites. A detailed kinetic analysis is used to model the variation in selectivity and to establish the turnover frequencies for the various reactions involved.