Thermogravimetry in tandem with mass spectrometry has been used to characterise the thermal decomposition of synthetic alunites of potassium, sodium and ammonium. Three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal decomposition of the three alunites is different. For NH4-alunite, an additional process of de-ammoniation is observed which occurs simultaneously with dehydration. Dehydroxylation takes place in a series of four steps. De-sulphation occurs for K-alunite at 680 degrees C in a single step in comparison with Na and NH4 alunites where de-sulphation is observed in a series of four steps. The temperature of desulphation is cation dependent. The thermal decomposition is not completed until around 800 degrees C. (c) 2006 Elsevier B.V. All rights reserved.