Dehydration in set dental stone was studied by employing specific heat measurements. Dehydration proceeds in two steps in air. In the case of bulk samples, especially those made with a lower water-powder ratio, three endothermic reaction stages are observed; a reaction of the dihydrate to the hemihydrate is apparently separated in two, one of which is not inherent. It is thought that the occurrence of a new peak is due to the difficulty for the dehydrated water to escape, depending on the porosity of the set stone.