Product yields measured by high-performance liquid chromatography from chemical trapping of Cl -, Br -, and H2O by an aggregate-bound arenediazonium ion in cetyltrialkylammonium halide {(CTRA)X, R ) Me, Et, n-Pr, and n-Bu; X ) Cl, Br}, micelles are used to estimate, simultaneously, interfacial counterion, Xm, and water, H2Om, concentrations as a function of [(CTRA)X] and tetramethylammonium halide concentrations, [(TMA)X]. The results are interpreted by using a two-site pseudophase model. Values of Xm and H2Om are estimated by assuming that when the product yields from reaction of a long-tail aggregatebound arenediazonium ion in micelles are the same as the product yields from reaction of its short chain analogue in an aqueous quaternary ammonium ion salt solution, then Xm ) [Xw] and H2Om ) [H2Ow] in those solutions. The results show that Xm and H2Om are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of Xm against [(CTRA)X] at a series of salt concentrations fall on separate curves. Xm increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of Xm and H2Om are essentially continuous functions of the aqueous counterion concentration, [Xw], at constant degree of micelle ionization, R. Three factors affect the shapes of these profiles. (a) An initial rapid increase in Xm is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [Xw], an incremental (slope of 1) increase in Xm with added counterion for (CTEA)Br, (CTPA)Br, and (CTBA)Br micelles is attributed to free movement counterions and co-ions between the interfacial region of the micelles (up to the micellar core) and the aqueous pseudophase. (c) Xm increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Brw] and ca. 1.2 M [Clw]. The increases in Xm are accompanied by concomitant decreases in H2Om. Specific salt-induced rod formation is attributed to dehydration (partial) and tight ion pair formation between surfactant headgroups and counterions. Phase separation of (CTBA)Br micelles in 0.5 M (TMA)Br occurs when the interfacial water concentration is too low to maintain micelle stability. The dependence of Xm on [Xw] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting Xm equal to the sum the aqueous and interfacial counterion concentrations.