The vapor composition, activities of the components, and Gibbs energies of melts in the Cs₂O-B₂O₃ system at a temperature of 1020 K are determined by the Knudsen effusion mass spectrometric method. The activities of the components and the Gibbs energies of melts in the Cs ₂O-B₂O₃ system at a temperature of 1020 are calculated in the framework of the approach based on the generalized lattice theory of associated solutions with the use of the melt model including vacancies. It is demonstrated that the calculated and experimental thermodynamic properties are in reasonable agreement.