The thermal decomposition of ettringite in the presence of water and impurity gypsum has been observed by energy-dispersive synchrotron X-ray diffraction and FTIR spectroscopy. The decomposition occurs rapidly at 114 °C to produce a monosulfate, with 0.96 nm interlayer spacing, and bassanite. The thermal expansion of ettringite from 70 to 110 °C is 42 × 10^–6 K^–1 in the a direction and 22 × 10^–6 K^–1 in the c direction. The gypsum acts as a transient intermediate in the decomposition. The mechanism of this rapid decomposition is discussed.