The dehydration of lithium sulphate monohydrate has been studied by isothermal thermogravimetry at 80 °C, under various partial pressures in water vapour. The kinetic curves are sigmoid, they are interpreted with a model involving nucleation and growth steps: the nucleation is supposed to occur at the surface of the grains and is followed by a rapid two-dimensional growth, the new phase developing towards the centre of the grains. The studied powder being composed of ellipsoidal particles, this geometry is taken into account in the model.