Processes of evaporation and thermodynamic properties of glasses and melts of the system MgO-B₂O₃-SiO₂ in the range of 1550-1800 K were studied for the first time by high-temperature mass-spectrometry. The resulting activities and Gibbs energies of components and also corresponding excess values point to negative deviations from ideal behavior and to the absence of immiscibility fields in glasses and melts of the system under study in the studied temperatures and concentration intervals.