The alkali nitrate mixtures are promising phase change materials (PCMs) in the temperature range (373−573) K for the development of thermal storage systems. In this context, their thermochemical properties and phase diagrams which provide useful thermal information are needed for their better use. In the present paper, the mixtures based on cesium nitrate, lithium nitrate, and sodium nitrate are studied by both experimental and optimization techniques. Thus, a critical analysis of thermodynamic data from literature sources of the phases in the CsNO3-LiNO3-NaNO3 ternary system including the pure components is performed. Therefore, reliable phase change data (enthalpy and temperature) of pure nitrates CsNO3, LiNO3 and NaNO3 are proposed. In addition, by means of differential thermal analysis (DTA), the binary CsNO3-LiNO3 system and two vertical sections (= 1 and = 0.2) in the ternary CsNO3-LiNO3-NaNO3 system are investigated. X-ray diffraction (XRD) technique is also used to analyze phases in the CsNO3-LiNO3 binary system at room temperature. The CsNO3-LiNO3 binary system is characterized by a congruent equimolar compound Cs0.5Li0.5NO3 which appears at (334 ± 2) K. Two eutectic reactions are found at (445 ± 2) K and (435 ± 2) K, respectively. The system exhibits also a plateau at (427 ± 2) K corresponding to the polymorphic transition of CsNO3. The CsNO3-LiNO3-NaNO3 ternary system shows two ternary eutectic reactions at (408 ± 2) K and (405 ± 2) K, respectively. Combining our results with experimental data available in the literature, an optimization of the thermodynamic parameters in the ternary system is performed with the help of Calphad approach. A reasonable agreement between the calculated results and the experimental data is obtained.