Thermal energy storage is an essential advantage of solar thermal power plants. The present paper focuses on latent heat storage using a phase change material (PCM). The paper lists literature and gives the current status of PCM work in the temperature range 200 to 350 °C. The system KNO3-NaNO3 is discussed in detail in terms of their thermo-physical properties in the liquid and solid phase. A comparison of literature data and own measurements for the density, heat capacity, thermal diffusivity and thermal conductivity is presented. Measurement results with the following methods are discussed: helium pycnometer, differential scanning calorimeter (DSC) and laser flash. Missing data of the thermal diffusivity and thermal conductivity are partly supplemented. Consistent thermo-physical properties in the liquid phase are presented.