We report a theoretical investigation on the role of water in the reaction between the hydroperoxyl radical and ozone. Due to the encouraging results obtained in our previous work on the HO 2 +O 3 reaction, and also because of the low computational cost involved, all calculations have employed the Kohn–Sham DFT formalism. In particular, functionals with a high percentage of exact exchange are utilized in order to attain a high accuracy at the saddle points for reaction. It is found that ozone reacts with the HO 2 ·H 2 O complex and that the classical barrier height of the oxygen-abstraction mechanism is now lower than the barrier to the hydrogen-abstraction mechanism. The implications of the results on the mechanism of the HO 2 +O 3 reaction are discussed.