Gypsum (CaSO4·2H2O) dehydration kinetics were investigated through energy dispersive X-ray diffraction (EDXD), under reduced pressure (100 Pa), in the temperature range 313–353 K. The process follows the JMAK (Johnson-Mehl-Avrami Kinetic) model. The fitting procedure of the Arrhenius expression provides an activation barrier of 18(2) kcal/mol. Under these experimental conditions, dehydration proceeds via a single-step conversion path gypsum → γ-anhydrite. Separate experiments of bassanite (CaSO4·0.5H2O) dehydration, carried out at similar conditions, indicate, as expected, a faster process γ-anhydrite being the final product. According to the structural relationships between bassanite and γ-anhydrite, dehydration should occur via the escape of water molecules along the axis of the channel (c axis) following a one-dimensional behaviour. Therefore, no Avrami model (which implies nucleation and growth of a new phase) can be applied.