The thermal decomposition of syngenite, K2Ca(SO4)(2).H2O, formed during the treatment of liquid manure has been studied by thermal,gravimetric analysis. differential scanning calorimetry, high temperature X-ray diffraction (XRD) and infrared emission spectroscopy (IES). Gypsum was found as a minor impurity resulting in a minor weight loss due to dehydration around 100 degreesC. The main endothermic dehydration and decomposition stage of syngenite to crystalline K2Ca2(SO4)(3) and amorphous K2SO4 is observed around 200 degreesC. The reaction involves a solid-state re-crystallisation, while water and the K2SO4 diffuse out of the existing lattice. The additional weight loss steps around 250 and 350 degreesC are probably due to presence of larger syngenite particles, which exhibit slower decomposition due to the slower diffusion of water and K2SO4 out of the crystal lattice. A minor endothermic sulphate loss around 450 degreesC is not due to the decomposition of syngenite or its products or of the gypsum impurity. The origin of this sulphate is not clear. (C) 2003 Published by Elsevier B.V.