A thermodynamic model is presented for the aqueous-air interface. For common organic solutes a relationship between the activity coefficients in the bulk solution and on the surface layer is deduced by using the concept of a surface monolayer. This relation has enabled development of all equation expressing the air-water surface adsorption coefficient as a function of bulk activity coefficients, surface tensions, molar volumes and vapour pressures of the adsorbing species. The data for 51 compounds, including alkanes, cycloalkanes, aromatics, halogenated hydrocarbons, alcohols, ethers and esters, and ketones were used in the study. (C) 2009 Elsevier Inc. All rights reserved.