The p-V-T equation of state of superhard boron suboxide B6O has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Grüneisen parameter δ T . Bulk modulus B 0 =181 GPa and its first pressure derivative B 0′ = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e., a = 1.4 × 10−5 K−1 and b = 5 × 10−9 K−2 as well as δ T = 6 have been included in the model. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to the volume at standard conditions, V 0. The fitted thermal expansion at 0.1 MPa well consistent with the experimental data as well as with ambient-pressure heat capacity c p , bulk modulus B 0 and δ T describing its evolution with volume and temperature have been discussed. The fitted value of Grüneisen parameter γ = 0.85 in agreement with previous empiric estimations for B6O and experimental values for other boron-rich solids has been reported.