Sulfurization of molybdenum trioxide by elemental sulfur through powder vaporization is a common method used for growth of molybdenum disulfide. Optimization of complexes between sulfur allotropes and molybdenum species using Density Functional Theory has revealed the molecular mechanism of sulfurization. Complete sulfurization of molybdenum trioxide to molybdenum disulfide requires at least three sets of nucleophilic addition-elimination reactions that generate the experimentally observed molybdenum oxysulfide intermediates along the reaction pathway. Each nucleophilic addition reaction of a sulfur allotrope to a molybdenum species gives rise to a molybdenum oxysulfide ring, which can dissociate into a more sulfurized molybdenum intermediate. At the typical growth temperatures used in powder vaporization, the equilibrium constants for these reactions are essentially unity. Thus, sulfurization is driven by excess sulfur and gas flow through the growth furnace.