A kinetic study has been undertaken of the thermal decomposition of naturally-occurring gypsum and pure calcium sulphate dihydrate. The reaction proceeds without any evidence of detectable melting. From X-ray diffraction analysis, it appears, that the decomposition reaction of both the natural gypsum and pure calcium sulphate dihydrate proceeds via the hemihydrate and CaSO4 . 0.15H(2)O to form anhydrite. Kinetic data for natural gypsum show that the reaction appears to proceed via the following : For the alpha-range 0 < alpha < 0.2, Jander’s three-dimensional diffusion equation is followed and an average activation energy of 88 +/- 18 kJ mol(-1) is found. For the alpha-range, 0.2 < alpha < 0.5, it is the first order with autocatalytic activation (the average activation energy is 97.9 +/- 4.3 kJ mol(-1)). In the alpha-range, 0.5 < alpha < 0.7, the d-dimensional Avrami-Erofe’ev equation fits (the average activation energy is 97.2 +/- 20.4 kJ mol(-1)) and in the 0.7 < alpha < 1 range, the two-dimensional Avrami-Erofe’ev equation (the average activation energy is 93.6 +/- 1.4 kJ mol(-1)). Kinetic data for pure calcium sulphate dihydrate show that the reaction seems to proceed via the following : For the alpha-range 0 < alpha < 0.1, Jander’s three-dimensional equation is followed and an average activation energy of 137. 2 +/- 24.5 kJ mol(-1) is found. In the alpha-range, 0.1 < alpha < 0.8, the d-dimensional Avrami-Erofe’ev equation (average activation energy is 107.3 +/- 2.4 kJ mol(-1)) fits and for 0.8 < alpha < 1, the first order kinetic model is used (average activation energy is 106.9 +/- 1.5 kJ mol(-1)).