The interaction of paraffin chains at aqueous surfaces is a major factor in defining the stability of interfacial structures such as monolayers and membranes formed by lipids and surfactants. Basic to understanding these structures are the intermolecular pair potentials as expressed in the two-dimensional virial coefficients. In this paper the available literature on the adsorption of normal alkanes at the vapor/ water interface is used to estimate second virial coefficients. Analysis of the data obtained by a vapor flow method for the C-5 to C-9 alkanes shows that the results are experimentally unreliable. Adsorption of the same range of alkanes has been studied by a chromatographic method, but uncertainties in the measurement of the column areas make estimates of second virial coefficients unsound. A set of reliable coefficients was obtained only for the lower homologs methane to butane. Calculations of the effective pair potentials for these paraffins at the water surface indicate that arguments given previously (J. Colloid Interface Sci. 1981, 81, 41) for chain-chain interactions in insoluble lipid monolayers account rather well for the estimated second virial coefficients without recourse to corrections for "hydrophobic" effects in the two-dimensional alkane gases.