The effect of calcium sulphate presented as soluble anhydrite (gamma - CaSO4) on the setting of gypsum plasters was investigated by measurement of the set strength and a microstructural examination of the set plasters. The anhydrite reduced the set strength by disrupting the interlocking crystal matrix of dihydrate needles. Additives, such as the nucleating agent calcium sulphate dihydrate and the accelerator potassium sulphate, increased the rate of the hydration reaction by increasing seeding and the rate of dissolution of calcium sulphate hemihydrate. The additives also alter the crystal habit of the dihydrate needles in the set plaster, resulting in reduced intercrystalline bonding and as a consequence reduced strength. The use of a novel in situ nucleation/acceleration system using KHSO4 and Ca(OH)(2), to generate microcrystalline dihydrate nuclei in the presence of K2SO4, resulted in a rapidly setting plaster with high strength.