Thermal analyses of synthetic and natural vivianite (Fe2+)(3)(PO4)(2)(.)8H(2)O) were determined using a high-resolution thermal analyser coupled to a mass spectrometer. Five dehydration weight loss steps were observed for the natural vivianite at 105, 138, 203, 272 and 437 degreesC. The first weight loss step involves the reaction (Fe2+)(3)(PO4)(2)(.)8H(2)O --> (Fe2+)(3)(PO4)(2) (.) 3H(2)O + 5H(2)O. The TGA/MS for the synthetic vivianite gave similar results to that of the natural sample. Mass spectrometry shows that water is lost up to 450 degreesC and after this temperature oxygen is lost. Changes in the structure of vivianite were followed using infrared emission spectroscopy. A model is proposed for the dehydration of vivianite.