Equilibrium relationships in the uranium-manganese-oxygen system in air have been investigated as a function of temperature. Various mixtures of U3O8 and MnO2 were heated and from the compositions reached at various temperatures, a ternary isobaric diagram was constructed showing the composition and phase changes occurring in air. Three ternary nonstoichiometric phases were found to exist over wide composition ranges with the nominal compositions MnU2O7, MnUO4, and MnU2O6. The MnU2O7 dissociated at 1040°C to MnUO4 and U3O8; MnUO4 was found to be stable only between 930 and 1175°C, while MnU2O6 exists above 1130°C.