Using the phase diagram and a simple solution model, a subliquidus miscibility gap was estimated for the B2O3‐SiO2system. The predicted coexistence boundary, showing a consolute temperature of 520°C, was flat and symmetrical and extended across the complete binary. Gradient furnace heat treatments of selected compositions in this system resulted in phase separation which corresponded closely to the coexistence boundary initially predicted. Calculations and preliminary experimental results indicate that temperatures and compositions exist wherein metastable three‐liquid immiscibility occurs in R20–B203‐SiO