We have calculated the structure of the complex between H2O and HO2NO2. The species H2O-HO2NO2 has a relatively large binding energy of 6.5 kcal mol(-1) at the B3LYP/6-311++G(3df,3pd) level of theory. Vibrational frequencies were also calculated. These data were used to calculate the equilibrium constant for the formation of the complex, as well as the rate constant for its dissociation. Using these calculations in conjunction with the Tree method, we computed a reaction rate constant for the HO2-H2O complex with NO2 and compared it to the rate constant for HO2 + NO2. The HO2-HO2NO2 complex is presented for a point of comparison.