Previous results have shown that the active surface in stabilized active sulfide catalysts is carbided. This fact led us to reconsider the influence of organosulfide agents in the activation of hydrotreatment catalysts. Structural and morphological consequences of dimethylsulfide treatment of (Co)/MoS2-based solids were studied. Results suggest that the electronic promotion of Mo by Co substantially influences the carbon replacement of sulfur atoms at the edges of molybdenum sulfide layers. Key words: MoS2; MoSxCy; CoMoS; CoMoC; hidryodesulfurization; dimethylsulfide