The thermodynamic properties and phase relation of an alkali metal-oxychloride system were examined; this is a basic model system of secondary fly ash generated in the volume reduction process of bottom ash and fly ash from incineration processes. The phase diagram for the NaCl–Na₂CO₃ system and its thermodynamic properties have been examined by a hot filament method and electromotive force (EMF) method using Na β-Al₂O₃ as a solid electrolyte from 500°C to 900°C. The liquidus lines and the eutectic temperature show good agreement with previous reports. Activities of Na₂CO₃ for the NaCl–Na₂CO₃ system obtained in this study are compared with the data from the phase diagram.