A convenient flotation technique for measuring the selectivity coefficient of counterions at surfactant solution/air interfaces is presented. The selectivities of Cl-, NO3-, I-, and ClO3- versus Br- are measured using cetyltrimethylammonium surfactants, and the selectivity of Cl- versus Br- is measured using cetylpyridinium surfactants. The selectivity coefficient of Br- over Cl- is observed to be independent of solution composition, surface excess, and surfactant head group type. The measured selectivity coefficients were found to be multiplicative. We show how our results differ from those obtained in micellar experiments and argue that selectivity coefficients measured by flotation may be interpreted as thermodynamic ion exchange coefficients.