Eight glass samples in the B2O3-SiO2 system with compositions from 20 to 90 mol% B2O3 were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B2O3 (g) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B2O3 in the liquid were determined at 1475 K. Thermodynamic activities of SiO2 and integral excess Gibbs energies were estimated from the thermodynamic activities of B2O3. The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.