The interfacial concentrations of Cl- and Br- in aqueous zwitterionic micelles were determined by chemical trapping by analyzing product yields from spontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion. Interfacial concentrations of Cl- and Br- in 3-(N-hexadecyl-N, N-dimethylammonio) propane sulfonate, HPS, micelles were higher than in bulk solutions prepared with Li+, Na+, Rb+, Cs+, tetramethylammonium (TMA(+)), Mg+2, and Ca+2 salts. In contrast, the interfacial concentrations of Cl- and Br- were generally lower than in bulk solution in hexadecylphosphoryl choline, HDPC, micelles for all salts except Mg+2 and Ca+2. In both HPS and HDPC micelles the interfacial concentration of Br-was higher than that of Cl-, showing that binding is anion selective. The cation had a large effect on the interfacial concentration of halide ions with HDPC micelles decreasing in the order Ca2+ > Mg2+ much greater than Li+ > Na+ > K+ > Cs+ > Rb+ much greater than TMA(+), These results are the first direct and extensive determination of local halide ion concentration at the surface of zwitterionic micelles, and they demonstrate that chemical trapping methodology will work in membranes at physiologically relevant salt concentrations.