The structure, vibrational spectrum? and binding energy of HO2 . H2O are predicted using ab initio molecular methods. Theory predicts that the HO2 . H2O complex is bound by 6.9 kcal mol(-1). The equilibrium structure and rotational constants of the complex have been determined. The OH stretching frequency of the HO2 is red-shifted by 296 cm(-1) from the isolated molecule. Its infrared intensity is also enhanced by 22 times that of the monomer.