Most salt hydrates, especially those proposed for thermal-energy-storage applications, melt incongruently. In static systems, this property often leads to differences between the enthalpy of fusion and enthalpy of solidification. By means of differential scanning calorimetry (DSC), these differences have been determined for several salt hydrates. For Na 2SO 4 · 10 H 2O, the enthalpy of solidification at or near the peritectic temperature is never more than 60% of the enthalpy of fusion; further cooling leads to a second phase transition at a temperature corresponding to eutectic melting of mixtures of ice and this hydrate. This asymmetrical melting and freezing behavior of Na 2SO 4 · 10 H 2O decreases its potential as an energy-storing medium and also limits its usefulness for temperature calibration of DSC instruments. Sodium pyrophosphate decahydrate, Na 4P 2O 7 · 10 H 2O, although in some ways a higher temperature analog of Na 2SO 4 · 10 H 2O, exhibited a smaller discrepancy between the enthalpies of fusion and of solidification; its relatively high transition temperature permits a more rapid solidification reaction than is the case for Na 2SO 4 · 10 H 2O. For Mg(NO 3) 2 · 6 H 2O, a congruently melting compound, the magnitude of Δ H of crystallization equalled Δ H of fusion, even when supercooling occurred; a solid-state transition at 73°C, with Δ H = 2.9 cal g −1, was detected for this hydrate. MgCl 2 · 6 H 2O, which melts almost congruently, exhibited no disparity between Δ H of crystallization and Δ H of fusion. CuSO 4 · 5 H 2O and Na 2B 4O 7 · 10 H 2O exhibited marked disparities. Na 2B 4O 7 · 10 H 2O formed metastable Na 2B 4O 7sd 5 H 2O at the phase transition; this was derived from the transition temperature and verified by relating the observed Δ H of transition to heats of hydration. Peritectic solidification of hydrates can be viewed as a dual process: crystallization from the liquid solution and reaction of the lower hydrate (or anhydrate) with the solution; where Δ H of solidification appears to be less in magnitude than the Δ H of fusion, the difference can be attributed to slower reaction rate between solution and the lower hydrate. New or previously unreported values for Δ H of fusion obtained in this study were, in cal g −1: Mg(NO 3) 2 · 6 H 2O, 36; Na 4P 2O 7 · 10 H 2O, 59; CuSO 4 · 5 H 2O, 32; Na 2B 4O 7 · 10 H 2O, 33.