The development of deep-UV second-harmonic generation spectroscopy (SHG) for measuring the strong charge transfer to solvent (CTTS) transitions characteristic of all stable aqueous anions has provided a powerful new probe of water interfaces. By employing suitable models, quantitative thermodynamic results have been obtained for a number of fundamental electrolytes, which are generally in good agreement with theoretical calculations. Details of the experiments and models are described and salient results supporting a novel mechanism for the selective adsorption of ions to the air/water interface are reviewed.