Ammonium paratungstate tetrahydrate (NH4)(10)[H2W12O42].4H(2)O (APT), a starting material in powder metallurgy of tungsten, has been subjected to a complex thermoanalytical study. The weight loss stages and the various heat effects have been followed by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) in flowing 10% H-2/Ar and 10% H-2/He atmospheres up to 600 degreesC. Meanwhile evolution of gaseous products has also been analyzed and monitored by both on-line coupled mass spectrometer (TG/DTA-MS) and infrared gas cell (TG-FTIR). In atmospheres containing 10% H-2, the evolution courses of H2O and NH3 traced by these EGA-methods and the TG/DTA curves up to 380 degreesC have been found quite similar to those of measured in air or in pure He. Effect of the reductive medium occurred above 380 degreesC, where solid products of intermediate temperatures have been structurally evaluated by both FTIR spectroscopy and powder X-ray diffraction (XRD). Based on significant changes in XRD pattems, the only small exothermic heat effect observed at 420-440 degreesC is assigned to the crystallization heat of a mixture of partially reduced tungsten oxide bronzes. (C) 2004 Elsevier B.V. All rights reserved.