A microscopic site-binding ion-exchange model is developed for the binding of counterions to surfactant-coated interfaces. This model is used to describe several features of effective or thermodynamic selectivity coefficients for ion binding at air/solution interfaces as determined by surface tension or ion flotation, which depend on the Gibbs surface excess. The dependence of selectivity on electrolyte concentration is derived, and theoretical predictions are compared with experiment. determination of specific binding constants for individual ions from experimental data. The model is used to explain differences between selectivity coefficients at micelle surfaces and air/solution interfaces using various techniques.