One of the important steps of tungsten (W) production is the thermal decomposition of ammonium paratungstate tetrahydrate (APT4, [(NH 4 ) 10 (H 2 W 12 O 42 )•4H 2 O]) which is an intermediate product of the process. In the literature, there are different findings on the thermal decomposition mechanism of APT4. The investigation of thermal decomposition kinetics of APT plays an important role in reactor and process design.In this study, the thermal decomposition mechanisms and reaction kinetics of APT4 were investigated by using thermogravimetric, differential thermal and mass spectrometric analyses (TGA/DTA/MS) under dry air and inert gas atmospheres. The characterizations of the intermediate and final products obtained in each thermal decomposition step were carried out by X-ray Powder Diffraction (XRD) and Fourier Transform-Infrared (FT-IR) analytical techniques. Decomposition reaction mechanisms were determined. Apparent activation energies were calculated for the second, third and fourth decomposition steps.