A theoretical investigation of the vibronic interactions between the X B-2(2) and B (2)A(1) states has been performed in order to explain the unusually low nu(5) and nu(6) vibrational energies of the fluoroformyloxyl radical (FCO2). The coupling constants have been determined within the adiabatic quadratic approach (AP2) in the general framework of the model of Koppel, Domcke and Cederbaum. All Hamiltonian matrix elements have been parameterized with ab initio calculations carried out using either the EOMIP-CCSD or UHF-CCSD(T) method. The calculated nu(5) and nu(6) fundamental energies agree very well with the experimental value. (c) 2013 Elsevier B.V. All rights reserved.