The alpha- and beta-forms of calcium sulphate hemihydrate, CaSO4.1/2H(2)O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO4.1/2H(2)O to CaSO4.2H(2)O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps : dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the alpha- and beta-forms with water again appeared to be due to their different physical states.