The sulfidation of crystalline m-WO3 and (WO3H2O)-H-. and the thermal decomposition of (NH4)(2)WO2S2 to an {WOS2} oxysulfide and (NH4)(2)WS4 to {WS3} were studied by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), thermogravimetrical analysis (TGA), and laser Raman spectroscopy. Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structures of crystalline m-WO3 and (WO3H2O)-H-.. The sulfidation reaction starts at low temperatures with a reduction of the crystalline oxides by reaction with H2S without incorporation of sulfur. Only after this reduction does an exchange of terminal O2-:ligands of the oxides for S2- take place. In subsequent W-S redox reactions S-2(2-) ligands form, which leads to oxysulfidic intermediates. The {WOS2} oxysulfide phase serves as a reference of structural features of oxysulfides. The presence of oxygen at bridging and terminal positions as well as of disulfide ligands seems to be a common structural property of oxysulfides. The decomposition experiments with (NH4)(2)WS4 show that the sulfidation of m-WO3 and (WO3H2O)-H-. does not proceed via {WS3}-type intermediates.