Data on the high temperature behavior of components of borosilicate glasses and melts are of great importance for the preparation and utilization of glasses in the electrovacuum industry, for obtaining gradient optical fibers, for incorporation of nuclear waste etc. Using high temperature mass spectrometry (Knudsen effusion method) the compositions and partial pressures of vapor over glasses and melts at 1273-1973 K have been studied in the B₂O₃-SiO₂ system and in the multicomponent borosilicate systems containing Na₂O, K₂O, Li₂O, CaO, MgO, Al₂O₃ and MoSi₂. It has been shown that the evaporation of various components forms different vapor compositions. However, B₂O₃, SiO and alkali metaborates and their dimers are found in the vapor over the systems studied. Activities of the components have been determined. This makes it possible to calculate Gibbs free energies in multicomponent systems by the semiempirical Kohler method using the data on equilibria in binary systems.