The role of HO3 as a temporary reservoir of atmospheric OH radicals remains an open question largely because of the considerable uncertainty in the value of the dissociation energy of the HO-O-2 bond (D-0) or, equivalently, the standard enthalpy of formation of HO3 (Delta H-f(-)). Using a supersonic flow apparatus, we have observed by means of laser-induced fluorescence the decay of OH radicals in the presence of O-2 at temperatures between 55.7 and 110.8 kelvin (K). Between 87.4 and 99.8 K, the OH concentration approached a nonzero value at long times, allowing equilibrium constants for the reaction with O-2 to be calculated. Using expressions for the equilibrium constant from classical and statistical thermodynamics, and values of partition functions and standard entropies calculated from spectroscopic data, we derived values of D-0 = (12.3 +/- 0.3) kilojoules per mole and Delta H-f(sic) (298 K) = (19.3 +/- 0.5) kilojoules per mole. The atmospheric implications of HO3 formation are therefore very slight.