For residential heating and cooling applications, the most promising heat storage phase-change materials (PCM's) are the inorganic salt hydrates. Most of the common salt hydrates segregate during cyclic freezing and melting, losing storage capacity. A class of hydrates, congruent melting materials, does not suffer this disadvantage. Four of these are now available commercially as bulk chemicals, formulated with nucleating additives to suppress supercooling. The properties of these PCM's are discussed.