Measurements have been made of the rate of dehydration of desulphurized gypsum with particle diameters in the range of 35-67 mum in a fluidized bed reactor. Experiments were carried out at bed temperatures of 100 to 170 degreesC. The fluidizing gases were air, with water vapour pressures of between 0.001 atm and 0.35 atm, and carbon dioxide. The results show that the dehydration under all these conditions can be modelled using the two dimensional Avrami-Erofe'ev expression. In the experiments extreme care was taken to ensure that the generation of reaction products did not significantly influence the reaction rate. At the calcination temperature of 140 degreesC, the value used in industrial applications, a unique correlation between the reaction rate and water vapour pressure is apparent irrespective of the desulphurized gypsum particle diameters. Under these conditions, it is also shown that a unique relation between conversion and reduced reaction time then follows.