Volatilization of glass components and formation of inhomogeneity in a glass melt during heating often result in striae, devitrification or fracture of the final glass products. Borosilicate glasses were known of their relatively high volatility at the temperatures of melting.Volatilization loss at 1410°C was measured by thermo-balance method on the ternary Na2O-B2O3-SiO2 glasses in the composition range of 0-40 Na2O, 0-40 B2O3 and 60-85 SiO2 in wt%. The glass batches were prepared using purified sand and reagent grade chemicals and were melted in a platinum crucible in electric or gas-fired furnace. For the weight loss measurements, the glasses were held in small platinum crucible suspended in a vertical electric furnace. Results were as follows:1) The amont of volatilization loss in a fixed time (for example, 1 or 6hr) and with fixed Na2O/B2O3 ratio decreased with increase of the SiO2 content, and for the constant SiO2 content the loss had its maximum at Na2O/B2O3 ratio slightly lower than 1. At these maxima, the losses were far greater than those of the corresponding Na2O- or B2O3-SiO2 binary glasses.2) In almost all the glasses, the loss increased linearly with time. at the initial periods of heating. Generally, however, volatilization rates decreased with time.3) Especially in glasses containing larger than 75wt% SiO2, crystallized surface layeres were formed in the course of the measurement, and in consequence, the volatilization rates decreased distinctly.4) Under the condition of the measurements, thermal convection was observed in the crucible. Contrary to the case of glasses with relatively high SiO2 contents, in glasses with low SiO2 contents thermal convection took place distinctly that the considerable mixing phenomenon was observed in the glass melts.5) One of the possible mechanisms which explain the results described in 1) and 2) is given in the following: At the initial periods of heating, volatilization rate is controlled by the evaporation of the volatile components from the melt to the atmosphere or by the flowing away of the gas atmosphere near the surface of the melt. Then, diffusion controlled mechanism or the effect of the lowering of the concentration of the volatile components in the melt gradually predominates, and lowers the volatilization rate.6) Quatitatively, the relation between composition an volatilization loss showed a good agreement with the results of kolykov measuring at 1200°C (ref. 5). Quantitatively, however, the loss was 10-100 times larger than those by kolykov owing to the differences of the mesuring temperatures. Under the assumption that the initial volatilization rate is proportionate to the vapour pressure of glasses, it could be inferred that phase separation in the glass melts occur in the composition range with relatively low Na2O contents.