Thermal dissociation of equimolar salt mixtures of copper (II) chloride with alkali metal chlorides (LiCl, KCl-LiCl, KCl-NaCl, RbCl, CsCl) was studied and the equilibrium constant of the reaction of copper chloride with chlorine was determined. It was found that the thermal stability of copper (II) chloride increases in the enumerated series of solvents. This is explained by intensification of complexing between copper (II) cations and chloride anions. A mechanism of thermal dissociation of copper (II) chloride in mixtures of alkali metal chlorides is discussed.