Abstract: Na.sub.2 HPO.sub.4.12H.sub.2 O is prevented from deteriorating as a heat storage medium after repeated cycles of heat storage and heat withdrawal resulting from precipitation of dehydrated forms of the salt by mixing thoroughly into the dodecahydrate material a small water content up to 5% of the mixture by weight, after which the material is heated above the melting point of the dodecahydrate and then cooled in a gas-tight container. Between 0.1 and 2% by weight of urea, and most preferably about 0.7% by weight thereof, may also be incorporated into the mixture, as by first dissolving the urea into the water to be added, in order to reduce the thermal expansion coefficient of the heat storage medium.