Phase equilibria at 25°C in the KNO 3 -Ca(NO 3 ) 2 -H 2 O system were studied by the isothermal sections method. The phase diagram of the system was found to feature six phase fields, three monovariant lines, and two invariant points, the latter corresponding to a eutonic solution saturated with KNO 3 and KNO 3 ⋅Ca(NO 3 ) 2 ⋅3H 2 O and to a peritonic solution in equilibrium with Ca(NO 3 ) 2 ⋅4H 2 O and KNO 3 ⋅Ca(NO 3 ) 2 ⋅3H 2 O. The structure of the double salt was determined by X-ray structural analysis; its melting temperature and heat of melting were determined by differential scanning calorimetry (DSC) as ∆Н m = 132.2 kJ/kg, and T m = 52.1°С. The density of salt melts was measured at 60, 80, and 85°C. The thermophysical parameters of the double salt allow us to regard it as a promising heat storage material.