The lowest energy electronic transitions in the weakly bound van der Waals complex of water and oxygen (H2O.O-2) are studied using ab initio methods. The vertical excitation energies for the two low-lying singlet states are calculated with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods, and are compared to those calculated in the oxygen molecule. The MRCI calculations predict blueshifts of about 150 and 250 cm(-1) for the transition frequencies on formation of the complex. These calculated shifts can provide assistance towards the spectroscopic identification of H2O.O-2. (C) 2003 American Institute of Physics.