Ukrainian sulfur-rich coal containing about 3.6 mass % of sulfur was studied. The desulfurization process was performed in the fluidized-bed reactor in dry (4 vol % of H2O vapor) and wet (30-70 vol % of H2O vapor) atmospheres in the temperature range of 350-450 degrees C. A significant influence of the water-vapor content in the reaction mixture on the sulfur removal during the oxidative desulfurization of the coal was observed especially at low temperatures. The extent of FeS2 oxidation, the main sulfur-containing compound, was studied by Mossbauer spectroscopy. The Mossbauer data revealed different iron-containing products (FeSO4 center dot nH(2)O, Fe2O3, Fe3O4, and Fe1-xS) formed in the course of the process carried out in dry and wet atmospheres. The promoting effect of water vapor on the pyrite transformation was observed especially at low temperatures. The mechanism of this promotion on the molecular scale was proposed.