The dehydration of lithium sulphate monohydrate has been studied by isothermal thermogravimetry at 80 degrees 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.