The kinetics and mechanism of the isothermal dehydration of gypsum ore were investigated. The gypsum ore was heated up to the dehydration temperature of the CaSO4 . 2H(2)O --> CaSO4 . 0(5)H(2)O + 15H(2)O reaction in a fluidized-bed reactor. Although the process was expected to be rather simple, the dehydration proved to include two reactions mechanisms, a first-order mechanism and a two dimension phase boundary mechanism. For low temperatures, around 100 degrees C, dehydration is dominated by the first-order reaction which seemed to consist of the external diffusion of water molecules evaporated from the surface of the solid. The main mechanism probably describes the conversion of calcium sulphate from the dehydrate into the hemihydrate form in the temperature range of 110 - 140 degrees C. An Arrhenius-type kinetic equation was proposed and the parameters, a pre-exponential factor of A = 4.10(3) and an activation energy of E = 42 kJ/mol, were calculated.1065253260