The decomposition behavior of ettringite, Ca6Al2(SO4)3(OH)12·26H2O, at different temperatures was studied by means of isothermal XRD experiments, in which the evolution of the solid is monitored as a function of time. The experiments were performed at 40, 50, 55, and 60 °C for a natural ettringite specimen. The experimental data were used to construct a temperature-transformation-time (TTT) diagram. Such a diagram enables the prediction of the reaction pathways during the transformation process. The decomposition behavior was also studied under nonisothermal conditions using thermogravimetry and differential scanning calorimetry, and the obtained results were correlated with the results of the XRD study. Finally, the transformation kinetics and the activation energy (Ea = 246.1 kJ·mol(-1)) of the reaction were estimated using the so-called "time to a given fraction" method. The temperature at which the initial transformation stage occurs (lower than 50 °C) indicates that ettringite cannot be considered a suitable host phase for the immobilization of radionuclides and other harmful elements, as is frequently proposed in the literature.