Abstract: 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.

Abstract: The heat storage material, which undergoes reproducible transition in the range 66.degree. to 74.degree. C., comprises sodium pyrophosphate decahydrate (Na.sub.4 P.sub.2 O.sub.7.10 H.sub.2 O) dispersed throughout an open cell polymeric matrix which is stable in an aqueous medium at 80.degree. C.In use (for example, for space heating) the material is alternately heated to above 80.degree. C. and cooled below its transition temperature with evolution of heat.