A reversible liquid/solid phase change composition comprising a mixture of hydrated CaCl2 and KCl in which the KCl modifies the semicongruently melting behavior of CaC12 6H20 to the extent that the mixture approaches congruent melting behavior.The composition preferably includes nucleating additives selected from barium and strontium salts to modify and suppress the supercooling properties of the liquid phase of the composition.The composition most preferably comprises an admixture of hydrated C...