Mass losses vary between 13.7 and 16.5% and heat of dehydration values between 377 and 420 J g(-1) for the dehydration of ground and mixed inhomogeneous gypsum samples, As for the dehydration of CaSO4 . 2H(2)O, the dehydration of synthetic gypsum proceeds via multi-step reactions, Using a heating rate of 5 degrees C min(-1), the very slow dehydration of CaSO4 . 2H(2)O and impurities in the gypsum samples start at temperatures lower than 95 degrees C. The main dehydration of the calcium sulphate dehydrate part of synthetic gypsum occurs between 95 and 170 degrees C (heating rate 5 degrees C min(-1)) and seems to proceed via a process with an activation energy of 392 +/- 100 kJ mol(-1) for alpha-values between 0 and 0.1, For alpha-values between 0.1 and 0.7, the reaction can be described by a first-order process with autocatalysis with an activation energy value of 100.5 +/- 1.2 kJ mol(-1), The third part of the reaction (0.7 < alpha < 1), up to temperatures of 180 degrees C, gives an activation energy value of 96 +/- 15 kJ mol(-1), Even at temperatures above 250 degrees C, some CaSO4 . O.15H(2)O was still observed, The dehydration of calcium hemihydrate seems to proceed via the formation of CaSO4 . O.15H(2)O.