Hydrogen fluoride readily associates with water vapor in the air to form the H2O-HF complex, which has optical absorption properties that are different from those of unassociated hydrogen fluoride. Consequently, to accurately quantify the total amount of hydrogen fluoride coming from an industrial stack or vent, for example, requires that the complexed amount of hydrogen fluoride be accounted for. To do this, the equilibrium constant for the association reaction is needed. Using statistical mechanical methods, Adebayo and coworkers [Adebayo SLA, AC Legon and DJ Millen. 1991. J. Chem. Soc. Faraday Trans. 87:443-447.] made an estimate of this equilibrium constant based on measured and estimated spectroscopic values for the complex. This value is Keq = 84(18) but has never been confirmed experimentally. In the research described herein, we have measured the equilibrium constant for the formation of H2O-HF by monitoring the decrease in the infrared absorption features of water vapor and hydrogen fluoride in mixtures of these two vapors in a temperature-regulated cell. Our experimental values for Keq at 25 °C, based on the diminution of the water vapor and HF transitions, are 77(46) and 51(24), respectively. The decay of the spectral features continues long after (hours) the two vapors are mixed in the gas cell. This suggests that other reactions (the growth of a film of hydrofluoric acid, perhaps) are taking place. While the experimental results agree reasonably well with the calculated equilibrium value of Adebayo et al., further improvements in experimental and analysis techniques are needed to reduce the uncertainties in the experimental results.