The stability of ettringite as function of temperature and water vapour pressure was studied on synthetic samples using X-ray diffraction, thermogravimetric analysis, sorption balance measurements and different calorimetry techniques. Crystalline ettringite has a water content per mol of Al2O3, n, varying from 30 to 32 H2O, while the amorphous product formed on drying it, metaettringite, has only 9 to 13 H2O. Very large hysteresis was observed during the desorption and absorption cycles. The total enthalpy values associated with the decomposition and reformation of ettringite were consistent and similar. During rehydration, different absorption stages were identified and quantified with sorption calorimetry. Due to the hysteresis it was not possible to model the absorption/desorption behaviour using the enthalpies determined by calorimetiy, so the values calculated with the van't Hoff equation were used. The area of performance of ettringite-based systems recommended in this work has been extended compared to previously reported data. (C) 2015 Elsevier Ltd. All rights reserved.