Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N-2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <-> HO2-H2O was determined to be K-2 = (5.2 +/- 3.2) x 10(-19) cm(3) molecule(-1) at 297 K. Substituting K-2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm(3) molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2 -H2O]/[HO2] was estimated from the value of K-2 to be 0.19 +/- 0.11.