The mechanism of the SO2 + HO2 reaction was studied theoretically for the first time. Three product channels were revealed, namely, O-2 + HOSO, O-2 + HSO2, and OH + SO3. The O-2 + HOSO channel dominates the reaction under combustion conditions. A five-member-ring complex [SO2-HO2] exists at the entrance of the reaction. The structure and binding energy (De and DO) of the SO2-HO2 complex have been calculated. In view of D-0 = 21.2 +/- 2.0 kJ mol(-1), the SO2-HO2 complex should be stable at low temperature. The infrared spectra and frequency shifts were calculated for both SO2-HO2 and SO2-HO2, and compared with the available experimental data. (c) 2005 Elsevier B.V. All rights reserved.