Abstract: The dehydration of gypsum CaSO₄.2H₂O has been studied, at negligible water vapour pressure, by in situ infrared (IR) spectroscopy and by thermogravimetry to determine whether intermediate phases (CaSO₄.nH₂O) exist, other than the hemihydrate with n=0.5, and also to compare the mechanism of the dehydration process when measured by two techniques with very different correlation lengths. Thermogravimetry shows an apparently continuous water loss with an activation energy of 90.3 kJ.mol⁻¹, with no changes in the activation energy as a function of the degree of dehydration. IR spectroscopy on the other hand, clearly shows the existence of three discrete phases, gypsum CaSO₄.2H₂O, hemihydrate CaSO₄.0.5H₂O and γ-CaSO₄, with nucleation of each successive phase as dehydration proceeds. There is no evidence to suggest the presence of phases with any intermediate water content.