[en] Using the methods of spectrophotometry of melts and of heterogeneous equilibrium, the equilibrium composition of the mixture of plutonium valent forms appearing in oxidation of Pu³⁺ by chlorine-oxygen gaseous mixtures in the melt of the eutectics NaCl-CsCl has been studied. The equilibrium constants of the reactions PuO₂²⁺+ Cl^- reversible PuO₂⁺+1/2 Clsub(2 (g)), PuO₂⁺+ClH- reversible PuOs (solid)+1/2 Clsub(2(g)) are determined. It is shown that at temperatUres above 700 deg C in the melt, besides PuO₂⁺ and PuO₂²⁺ oxyions a new oxygenous form of plutonium, very likely PuO²⁺, appears