The surface tension determined by the sessile drop technique increases markedly as B2O3 is replaced by SiO2 at constant mole fraction Na2O (N(Na2O) = 0.33). Addition of SiO2 to glasses of constant Na:B ratio (Na:B = 0.5) results in a slight increase in surface tension in the range N(SiO2) = 0 to 0.4.