Heat storage materials

The material comprises at least one hydrated compound which undergoes reversible transformation to a less hydrated form and a fusion temperature-depressing salt, both dispersed and suspended throughout a water-swollen cross-linked polymer hydrogel (the hydrogel being as in European Pat. Nos. 99 and 11411). The improvement is that the fusion temperature-depressing salt is present in 50 to 75% of the eutectic amount; this substantially reduces thermal hysteresis. In a modification of the invention, water is present in excess of the amount required to fully hydrate the compound, in which case the amount of fusion temperature-depressing salt may be 50 to 90% by weight of the eutectic amount.