The composition of the vapor and the activities of cesium metaborate CsBO₂ in glass melts of the Cs₂O-B₂O ₃-SiO₂ system at a temperature of 1020 K are determined using high-temperature mass spectrometry. The activities of the individual oxides Cs₂O, B₂O₃, and SiO₂ in glass melts of the Cs₂O-B₂O₃-SiO₂ system at a temperature of 1020 K are calculated by the Wagner method from the obtained values of the activities of CsBO₂. The integral and excess Gibbs energies are determined.