Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al2O3and up to 1620°C for pure molten B2O3. Large expansion coefficient decreases and a slight activation energy increase for B2O3above 1400°C suggested a tightening of its structure. The addition of Al2O3reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO2additions) and the increased expansion coefficients accompanying Al2O3additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO2and GeO2additions at 1300°C. Microclustering of aluminum‐bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO2and G