The lowest-energy electronic transitions in the hydroxyl radical and the hydrogen bound complex H2O.HO have been studied using ab initio methods. We have used the complete active-space self-consistent field and multireference configuration interaction (MRCI) methods to calculate vertical excitation energies and oscillator strengths. At the MRCI level the lowest-lying (2)Sigma(+)<--(2)Pi electronic transition is redshifted by about 2500 cm(-1) upon formation of the H2O.HO complex. We propose that this transition could be used to identify the complex in the gas phase, which in turn could be used to examine the role of H2O.HO in atmospheric reactions. (C) 2004 American Institute of Physics.