The thermal decomposition of ammonium paratungstate tetrahydrate (APT center dot 4H(2)O), (NH4)(10)[H2W12O42]center dot 4H(2)O, in air to tungsten trioxide, WO3, was investigated under non-isothermal conditions using thermal analysis coupled on-line by a Skimmero system to a quadrupole mass spectrometer (TA-MS) for evolved gas analysis (EGA), with X-ray powder diffraction (XRD), with Fourier transform infrared (FT-IR) and Raman spectroscopy. The decomposition is characterized by three endothermic and one exothermic step. The first endothermic step comprises the release of crystal water resulting in the formation of crystal-water-free ammonium paratungstate. During the second endothermic step ammonia is released leaving behind ammonium hydrogen paratungstate, (NH4)(6)H-4[H2W12O42]. The presence of protons in the "roasted" APT was supported by ex situ and in situ FT-IR spectroscopy (vibration bands at 1100 and 2160 cm(-1)). Ammonium hydrogen paratungstate is the precursor for the metatungstate anion, [H(2)W(12)o(40)](6-) which is formed during the digestion of "roasted" APT with hot water. (C) 2008 Elsevier B.V. All rights reserved.