Boron suboxide B 6 O 1−x was synthesized from a mixture of amorphous boron or α-rhombohedral boron and B 2 O 3 in excess (nominal composition B 3.33 O). The use of α-boron resulted in a more completely crystallized boron suboxide, which was utilized to study the homogeneity range of the suboxide in the temperature range 1250 °C–1450 °C. The oxygen content of solid B 6 O 1−x was determined from Rietvelt analysis of X-ray powder diffraction intensities. The oxygen content decreased from 12.3 at% at 1250 °C to 11.6 at% at 1450 °C. It is probable that stoichiometric B 6 O (14.3 at% oxygen) can only be prepared at pressures considerably higher than ambient pressure.