Abstract: A heat storage medium includes a halide or sulfate salt of a divalent or trivalent metal which salt has hydrated and dehydrated states with high heat of hydration. The medium is dehydrated by heating it with heat derived from a natural source such as solar or geothermal energy or from industrial waste heat and the evolved water removed from the medium. The heat is recovered by rehydrating the medium by the addition of water. The medium may be a porous mass, a slurry of the salt in a water immiscible liquid or a suspension of the salt in a gas and it may be dehydrated by source heating the slurry and separating the water or by passing a source heated gas or immiscible liquid through the medium and the heat may be released from the medium by injecting water into the slurry or by passing a water carrying gas or immiscible liquid through the permeable mass. The storage and recovery procedure may be continuous or batch with the medium being recycled.