The eutectic Na2CO3-NaCl molten salt was investigated as a new high temperature phase change material for solar thermal energy storage. The composition of the eutectic binary salt was determined with the aid of FactSage software and its thermophysical properties were investigated using a Simultaneous Thermal Analyzer, X-Ray Diffraction and a Scanning Electron Microscope. Eutectic Na2CO3-NaCl salt shows higher measurement values in a CO2 atmosphere than these in a N2 atmosphere in terms of heats of both fusion and solidification. Thermal stability analysis indicates that the eutectic molten salt has higher thermal stability in a CO2 environment without weight loss at temperatures below 700 °C compared with 0.51% weight loss at the melting point around 640 °C in a N2 atmosphere. The weight loss observed in the latter, is most likely to be due to the salt’s decomposition at high temperature. The thermophysical properties of the salt such as melting temperature, heats of both fusion and solidification, as well as the phase identification and phase morphology varied slightly after 100, 200 and 300 thermal cycle tests. Therefore, the eutectic Na2CO3-NaCl salt has a good thermal and phase stability. It therefore is a promising high temperature phase change material when used in a CO2 environment.