Rationale: Cesium and molybdenum are fission products of uranium dioxide fuel in nuclear reactors, which interact with each other depending on the oxygen potential of the fuel. This leads to formation of various compounds of the Cs₂O–MoO₃ system, which are exposed to high temperatures during operation of a reactor or a severe accident at a nuclear power plant. This is why the study of the vaporization and thermodynamics of compounds in the Cs₂O–MoO₃ system is important. Methods: Synthesis of the compounds in the Cs₂O–MoO₃ system was carried out by sintering Cs₂MoO₄ and MoO₃. Characterization of the samples was accomplished with the use of XRD, TGA/DSC/DTA, IR spectroscopy, and ICP emission spectroscopy. Vaporization of the samples under study was carried out from a platinum effusion cell using an MS-1301 mass spectrometer developed for high-temperature studies of low-volatility substances. Results: The temperature dependences of partial pressures of vapor species were determined over pure MoO₃ and Cs₂MoO₄ in the ranges 870–1000 K and 1030–1198 K, respectively. MoO₃, Mo₂O₆, Mo₃O₉, Mo₄O₁₂, and Mo₅O₁₅ were shown to be the main vapor species over the Cs₂O–MoO₃ system in the temperature range 850–1020 K. The component activities, Gibbs energies of mixing, and excess Gibbs energies were obtained as functions of the component concentration at 900, 950, and 1000 K. Conclusions: The thermodynamic properties of the Cs₂O–MoO₃ system found in the study evidenced negative deviations from ideality. The MoO₃ and Cs₂MoO₄ partial molar enthalpies of mixing, the Cs₂MoO₄ partial vaporization enthalpy, and the total enthalpy of mixing in the Cs₂O–MoO₃ system at 1000 K were obtained for the first time.