The sulfidation of crystalline m-WO3 and WO3.H2O and the thermal decomposition of (NH4)(2)WO2S2 to an {WOS2} oxysulfide were studied by means of X-ray powder diffraction, quick extended X-ray absorption fine structure, and infrared emission spectroscopy. Several of the basic steps in the sulfidation reactions of m-WO3 and WO3.H2O were resolved and explained in terms of the structures of the oxides. In both cases, the sulfidation reaction follows the same general route, starting at low temperatures with the reduction of the crystalline oxides by H2S to give hydrogen tungsten bronzes. Only after this reduction can sulfur be incorporated into the oxide structure. At higher temperatures the hydrogen tungsten bronzes transform into shear oxides. Oxygen-sulfur exchange processes on the surface proceed at a higher rate, and shear oxysulfide pleases form. Further incorporation of sulfur leads to oxysulfide intermediates with higher S:W ratios. W-S redox reactions eventually transform the oxysulfidic phase into WS2.