Structural parameters, harmonic frequencies, and force constants of B2F4were obtained in internal coordinates from ab initio RHF calculations with the 6-311G* basis set. The results obtained are used to calculate the Coriolis coupling constants and the inertial defect for10B2F4and11B2F4and to compare these values with those obtained by using the autoconsistency method. Both the ab initio and the iterative force fields, despite some significant differences, reproduce the vibrational wave numbers equally well and yield nearly the same Coriolis coupling constants and inertial defects. The calculated inertial defects for10B2F4and11B2F4are -0.942 and -0.919 amu Å2, respectively.Key words: Coriolis coupling constants, inertial defect, B2F4, harmonic frequencies, force constants.