Iron promotes the interaction between S-2 and Mo(110). The exposure of Mo(110) to large amounts of S-2 produces only a chemisorbed layer of sulfur, without forming molybdenum sulfides. The sulfidation of Mo occurs after exposing Fe/Mo(110) surfaces to S-2 gas. Fe promotes the rate of formation of molybdenum sulfides by transferring charge to Mo (favoring in this way an electrophilic attack of S on Mo) and by changing the structure of the surface (making the penetration of S into the bulk of the sample easier). The thermal stabilities of the MoSx/Mo(110) and FeSy/MoSx/Mo(110) systems are comparable, with the bimetallic sulfide showing a somewhat lower decomposition temperature (similar to 1200 K). The behavior of FeMoS catalysts in hydrodesulfurization processes is discussed in light of these results.