The structural changes occurring during the thermal decomposition of ammonium tetrathiotungstate to form amorphous WS ₃ and poorly crystalline WS ₂ have been studied in situ using combined extended X-ray absorption fine structure (EXAFS) spectroscopy at the tungsten L _III -edge and X-ray diffraction. Data collected during isothermal decomposition at both 135 and 150 °C show that ammonium tetrathiotungstate decomposes to produce amorphous WS ₃ without the formation of any intermediate phases. Decay curves of starting material and growth curves of product derived from both X-ray absorption data and diffraction data show that the two-phase approach to data analysis is appropriate. The fit of various simple kinetic models to the thermal decay curves is tested and it is found that the Prout-Tompkins expression describes the process very well. Heating ammonium tetrathiotungstate from 100 to 350 °C produces first amorphous WS ₃ and finally disordered WS ₂ , a material of catalytic interest. The EXAFS data of this freshly prepared WS ₂ are compared to those of crystalline 2H-WS ₂ . Debye-Waller factors are increased and occupation numbers are significantly reduced from those in the crystalline material for all atomic shells. This behaviour is compared to previous results obtained from poorly crystalline molybdenum disulfide and possible structural models suggested to account for the results of the first EXAFS study of the disordered WS ₂ .