Surface-active amphiphiles aggregate spontaneously in water to form association colloids such as micelles, microemulsions, and vesicles. The hydrophobic effect drives aggregation, but the opposing forces that provide balance and determine equilibrium morphologies are not understood, in particular, how specific ion effects, which often follow a Hofmeister series, affect the properties of association colloids. We have harnessed the competitive trapping of arenediazonium ions by weakly basic nucleophiles such as halide counterions, anionic headgroups, alcohols, urea, and water, to estimate their concentrations in the interfacial regions of association colloids from reaction product yields. In the chemical trapping method, product yields are proportional to the concentrations of water and other nucleophiles within the interfacial region, not their stoichiometric concentrations in solution. Changes in the balance of forces controlling aggregate structure are reflected in changes in interfacial concentrations of water and other components in association colloids as reported by the chemical trapping method. Significant changes in interfacial water and counterion concentrations are observed during structural transitions. Specific ion effects on sphere-to-rod transitions of cationic amphiphiles are interpreted in terms of the strengths of headgroup and counterion pairing and ion hydration interactions. Trapping results also provide important information on interfacial compositions of microemulsions, vesicles, nonionic micelles and macroemulsions, reverse micelles, micelles in aqueous urea, and anionic polyelectrolytes. Identifying relationships between aggregate morphology and interfacial composition by chemical trapping has just begun.