The phase equilibria in the Cs₂O-B₂O ₃-SiO₂ system are investigated in the temperature range 873-1073 K by the annealing-quenching technique, high-temperature microscopy, and X-ray powder diffraction analysis. The vapor composition, the activities of the components, and the Gibbs energies of glasses and melts in the Cs ₂O-B₂O₃-SiO₂ system are determined at a temperature of 1020 K. It is demonstrated that glasses and melts in the Cs₂O • B₂O₃-Cs₂O • SiO₂ system are characterized by positive deviations of the CsBO ₂ activities and negative deviations of the Cs₂O • SiO₂ activities from ideal behavior. As a consequence, the deviations of the Gibbs energies from ideal behavior change their sign.