We report a systematic investigation of the adiabatic electron affinities (EA) of BO, NO and O-2 using a specifically adapted massively parallel version of the configuration-selective configuration interaction method (MRCI) which permits the explicit treatment of the most important triple and quadruple excitations (TQ). We find the EA(BO) = 2.52 eV, EA(NO) = 0.015 eV and EA(O-2) = 0-39 eV in good agreement with experimental results. We elucidate the physical mechanism that is responsible for the failure of standard MRCI to quantitatively describe the EA of oxygen containing molecules. (C) 2003 Elsevier Science B.V. All rights reserved.