Thermal energy storage material

A thermal energy storage material comprising at least one hydrated inorganic salt having a transition temperature to the anhydrous or a less hydrated form in the range 10° to 100° C. (for example, sodium sulphate decahydrate), the salt being dispersed and suspended in a water-insoluble hydrogel formed from a water-soluble synthetic polymer having pendant carboxylic or sulphonic acid groups cross-linked with cations of a polyvalent metal (for example, aluminium or magnesium).