Abstract Thermogravimetry combined with evolved gas mass spectrometry has been used to ascertain the stability of the soil minerals destinezite and diadochite Fe3+2(PO4)(SO4)(OH)·6H2O. These two minerals are identical except for their morphology. Diadochite is amorphous whereas destinezite is crystalline. Both minerals are found in soils. It is important to understand the stability of these minerals because soils are subject to bush fires especially in Australia. The thermal analysis patterns of the two minerals are similar but not identical. Subtle differences are observed in the DTG patterns. For destinezite, two DTG peaks are observed at 129 and 182 °C attributed to the loss of hydration water, whereas only a broad peak with maximum at 84 °C is observed for diadochite. Higher temperature mass losses at 685 °C for destinezite and 655 °C for diadochite, are due to sulphate decomposition, based upon the ion current curves. This research has shown that at low temperatures the minerals are stable but at high temperatures, as might be experienced in a severe bush fire, the minerals decompose.