Structural transformations and the mechanism of the operation of molybdenum-containing catalysts under hydrogenation conditions have been studied by chemical thermodynamics and quantum chemistry methods, as well as the role of sulfur compounds in this process. It has been shown that molybdenum disulfide, an effective hydrogenation catalyst, is produced via the reaction of molybdenum oxide with hydrogen sulfide, not elemental sulfur. It has been concluded that the introduction of elemental sulfur (which is subsequently converted into hydrogen sulfide) or an organic sulfur compound into the reaction system, on one hand, facilitates the transformation of molybdenum oxides into MoS(2) and, on the other hand, yields the effective reagent H(2)S, which takes part in the disproportionation reactions involving hydrogen transfer.