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, X,, 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 X-m and H2O, are estimated by assuming that when the product yields from reaction of a long-tail aggregate-bound 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 X-m = [X-w] and H2Om = [H2Ow] in those solutions. The results show that X-m and H2Om are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of X-m against [(CTRA)X] at a series of salt concentrations fall on separate curves. X-m increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of X-m and H2O, are essentially continuous functions of the aqueous counterion concentration, [X-w], at constant degree of micelle ionization, alpha. Three factors affect the shapes of these profiles. (a) An initial rapid increase in X-m is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [X-w], an incremental (slope of 1) increase in X-m 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) X-m increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Br-w] and ca. 1.2 M [Cl-w]. The increases in X-m 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 X-m on [X-w] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting X-m equal to the sum the aqueous and interfacial counterion concentrations.