The phase equilibria in the Cs 2 O-B 2 O 3 -SiO 2 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 2 O-B 2 O 3 -SiO 2 system are determined at a temperature of 1020 K. It is demonstrated that glasses and melts in the Cs 2 O · B 2 O 3 -Cs 2 O · SiO 2 system are characterized by positive deviations of the CsBO 2 activities and negative deviations of the Cs 2 O · SiO 2 activities from ideal behavior. As a consequence, the deviations of the Gibbs energies from ideal behavior change their sign.