The interaction of hydrocarbon chains at aqueous interfaces is a major factor in defining the interfacial structures of monolayers and membranes formed by surfactants. The intermolecular pair potentials which can be expressed in the two-dimensional second virial coefficients (B-2(T)) are basic to understanding these structures. In this report, measurements of the adsorption of hexane vapor on water as a function of the partial vapor pressure are reported for a static system in which the vapor pressure is controlled by mixing liquid hexane with either hexadecane or squalane. The B-2(T) for hexane at the water/vapor interface is approximately estimated at one temperature. The results confirm that the extensive published data on the adsorption of alkanes on water obtained in a vapor now system are incorrect, presumably due to inadequate vapor saturation.