Thermal analysis was applied to determine the liquidus temperatures in the BaCl2-MCI-BaO systems, where M - alkali metal. The performed studies of caloric effects at melting of chloride eutectic with barium oxide additions proved the chemical mechanism of barium oxide dissolution in molten chloride eutectic. The X-ray patterns of frozen fusion after the experiment illustrate that Ba4OCl6 is present in the solid phase. The essential composition changes of barium containing chloride melt after the BaO additions induce the considerable growth of liquidus temperatures.