AbstractCompacts made from powdered material are used in an effort to explain anomalous results obtained from the isothermal uptake of water vapour by various types of dehydrated gypsum. In the sorption isotherms, a sharp discontinuity occurred at R.H. between 0·25 and 0·40 and a secondary hysteresis was observed. Sorption of methanol on these materials was also measured, and results were compared with those obtained with water.The determination of the expansion isotherm enabled an application of the Gibbs adsorption equation to be made, which led to a method by which it was possible to differentiate between hemihydrate and adsorbed water and thus allow a calculation of a B.E.T. surface area from the water isotherm for the material.From this calculation and others obtained by use of a modified Kelvin equation, it was concluded that the anomalies noted above were due to sites on the surface of the plaster that required an activation energy for ‘sorption’ to occur. The generation of these sites was considered dependent on the method of preparation of the anhydrous calcium sulphate. The water attached to these sites would thus be termed ‘chemisorbed’ water.