The gas-phase reaction HCO + O-2 --> CO + HO2 has been investigated by means of ab initio calculations. The mechanism can proceed through either a direct hydrogen abstraction or addition Of 02 to the formyl radical. The energy barriers calculated at the QCISD(T)/6-311G(2df,2p) level of theory upon QCISD/6-311G(d,p) optimized structures are, respectively, of 2.98 kcal mol(-1) for the direct abstraction and of 2.26 kcal mol(-1) for the addition. Thus, the results obtained show that there is not a dominant pathway in the HCO + O-2 reaction under atmospheric conditions of temperature and pressure. (C) 2003 Elsevier Science B.V. All rights reserved.