A theoretical study of the HBN radical in the X (2)Pi and A (2)Sigma(+) states, taking into account vibronic coupling effects, is reported. The lowest (1 (2)A('),1 (2)A('),2 (2)A(')) potential energy surfaces (PES) of the HBN-BNH system have been studied to identify all stationary points. The HBN minimum was found to be 20.0 kcal/mol above BNH, with an isomerization barrier of approximate to11 000 cm(-1) on the 1 (2)A(') surface. For the HBN isomer, accurate near-equilibrium three-dimensional diabatic PESs for the ground X (2)Pi (1 (2)A('),1 (2)A(')) and first excited A (2)Sigma(+) (2 (2)A(')) electronic states have been calculated at the multireference configuration interaction level of theory, with extended basis set. A vibronic coupling between bend and BN stretch, analogous to that found in the isoelectronic C2H and HCN+ radicals, has been found to take place due to the crossing of the X (2)Pi and A (2)Sigma(+) states at energies close to 11 000 cm(-1). Vibronic energy levels of HBN and DBN have been calculated variationally using a previously developed method [Carter , Mol. Phys. 98, 1967 (2000)] suitable for three-atomic molecules showing three-state vibronic interactions. Energy levels of Sigma and Pi symmetry up to 10 000 cm(-1) for HBN, and 8800 cm(-1) for DBN, are reported. It is shown that due to the high-energy surface crossing, the vibronic interaction becomes non-negligible only for levels above 8500 cm(-1). For all levels, Renner-Teller effects and Fermi resonances are analyzed. (C) 2003 American Institute of Physics.