Samples of a nearly eutectoid mixture of 60 mole % BaSO~ --40 mole % CaSO~, a highly effective system for the selective sorption of strontium, were studied using the DTA method. The effects of the time of thermal treatment and the number of thermal cycles of two different types on the homogenisation degree and thus on the formation of the metastable barium --calcium s~lphate were examined. The conditions of the thermal decomposition of the metastable sulphate, the activation energy and the heat released during this process were estimated.