A previous study of the proposed intermediate chain free radical, HOOC.=O, in the reaction HO2 + CO --> OH + CO2 is refined and extended. Standard ab initio methods are used to establish transition states and their energies and geometries. Confidence for the methods is reinforced by finding that our value of Delta H degrees(298) = 242.7 kJ for reactants --> products is very close to Delta H degrees(298) = 246.1 +/- 8.4 kJ obtained from thermochemical tables. We obtain an energy for the transition state to form the radical of 92.9 kJ mol(-1), in good agreement with the experimental value of 96.0 kJ mol(-1) derived from high-temperature data. Part of this high activation energy comes from the strength of the O-O bond in HO2, which may be stabilized by three-electron bond resonance. The ground state HOOC.=O radical is at 48.5 kJ mol(-1) and the transition state from radical to products is at 79.8 kJ mol(-1) above the reactants, so once the intermediate is formed, an activation energy of only 31.3 kJ mol(-1) is required to form the products of the reaction. A study of the isomeric bicarbonate radical HOC(=O)O-. indicates that it is not a direct intermediate and that it is unlikely that it is involved in the reaction. For this radical two structures, anti and syn, have been identified, and the energies and structures for transition states for each to yield products are established. For the radicals we obtain Delta H degrees(f(298))) = -62.4 (chain), -353.8 (syn-bicarbonate), and -338.4 (anti-bicarbonate) kJ mol(-1).