Complex formation in the interaction of HO2 with H2O was studied using Fourier transform infrared absorption spectroscopy. Studies were carried out between 230 and 298 K in the presence of excess H2O. On the basis of measurements of HO2 disappearance, equilibrium constants (K-c) for the formation of the complex were determined to be 1.7 x 10(-16), 4.1 x 10(-17), and less than or equal to 4 x 10(-18) cm(3) molecule(-1) at 230, 250, and 298 K, respectively. The enthalpy for the reaction forming the complex was determined to be -36 +/- 16 kJ mol(-1). This is in good agreement with previous estimates made from kinetic measurements and theoretical calculations.