The setting process of gypsum plasters has been followed using strength testing, acoustic techniques and scanning electron microscopy. Gypsum plasters were found to set in three stages : formation of the calcium sulphate dihydrate crystal matrix followed by relief of internal stress, and finally evaporation of excess water. The alpha-calcium sulphate hemihydrate set faster than the beta-form, giving a higher initial and final set strength. The set alpha-plaster consisted of relatively short stubby dihydrate needles with a greater degree of intercrystalline bonding, resulting in greater strength. The microstructure of set alpha-plaster appeared to be the result of a slower precipitation rate of dihydrate during the hydration reaction of hemihydrate.