Ab initio gradient calculations of the geometry and vibrational force field of B 2O 3 have been carried out at the double zeta plus polarization level of the RHF method, with the value obtained for the inversion barrier being checked with the MC—SCF technique. B 2O 3 is a planar molecule of C 2V symmetry with equilibrium BO and BO bond lengths of 1.33 and 1.20 Å, respectively. The central angle of 136° is extremely flexible, with an inversion barrier of only 2.4 kcal mol −1 at the linear geometry. The OBO group is bent by 2°. The complete quadratic force field has been determined along with the most important cubic constants, and the vibrational spectrum has been predicted. The main features of the structure obtained in a recent electron diffraction study are confirmed, but the vibrational force field resulting from the analysis of that work appears to be seriously flawed. The expected vibrational frequencies and the intensifies of the infrared transitions are also computed and compared with the limited experimental data presently available.