The influence of a single water molecule on the gas-phase reactivity of HO2 radical has been investigated by theoretical studying the reaction of SO2 with the HO2 radical and the HO2 center dot H2O radical complex, respectively. The former leads to the formation of the HSO4 radical, while in the latter reaction the water slightly accelerates the SO2 + HO2 reaction to form HSO4 center dot H2O radical complex and the HO2 acts as a positive catalyst for the formation H2SO3 center dot HO2 complex in the SO2 + H2O reaction, respectively. The rate constants at 298 K are calculated using transition state theory. (C) 2014 Elsevier B.V. All rights reserved.