Complex formation between HO2 and H2O and kinetics of the HO2-H2O complex were investigated using near-infrared two-tone frequency modulation spectroscopy at 250--350 K and 50 torr with N2 diluent. From the depletion of the HO2 signal in the presence of water, the equilibrium constant of the HO2-H2O complex formation was estimated. The stabilized energy of the HO2-H2O complex formation was estimated to be 31 ¿ 4 kJ mol-1, and it was suggested that the complex has two hydrogen bonds. Using the equilibrium constant obtained from spectral measurements, the rate constant of the reaction HO2 + HO2-H2O was estimated at 297--350 K and 50 torr with N2 diluent. Weak dependence of the rate constant on temperature indicated that the temperature dependence of water effects on HO2 self-reaction kinetics was mainly controlled by that of the equilibrium constant of the HO2-H2O complex formation. Altitude profiles of HO2-H2O complexes and HO2 self-reaction rate were also estimated in the northwest Pacific Ocean. The importance of HO2-H2O complexes in the lower troposphere was suggested.