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28 pages of text including references and figure legends, 5 figures and 1 table 99 words in abstract and 46981 total characters in the paper Submitted to PNAS Contrasting current textbook descriptions, a consistent picture of sub-stantial concentration enhancement of highly polarizable anions at the sur-face of aqueous electrolyte solutions is emerging. Such enhancement may have important implications for chemistry occurring on aqueous aerosols and ocean surfaces. Here we present a combined experimental and theoret-ical investigation of the liquid/air interface of aqueous sodium thiocyanide at varying salt concentrations. Normalized second harmonic generation intensities fitted to Langmuir isotherms yield a Gibbs free energy of ad-sorption of-1.80 kcal/mol. These results are in accord with molecular dynamics simulations in slab geometry, which predict an appreciable sur-face enhancement of SCN−. 1