Density (and some viscosity) data are presented for binary sodium borate melts containing as much as 60 mole % Na2O and for ternary sodium silicoborate melts with B/Si<2.0 between 1000°C and 1300°C. The high‐temperature partial molar volume analysis of the binary sodium borate melts reveals about 50% BO4tetrahedra at the 40 mole % Na2O composition, in agreement with recent NMR estimates for the binary glasses. No “boron anomaly” was found near 18 mole % Na2O at high temperature. The synthetic partial molar volume model that agrees best with experiment for all ternary melts studied involves the presence of some BO4tetrahedra, the percentage of which varies with composition. This ternary model involves a high degree of internal consistency. No tendency toward extensive micro‐immiscibility was observed for ternary melts near the SiO2·