Cyclization of 2-(3-bromopropyloxy)phenoxide ion (PhBr7) in micelles is a model for S(N)2 reactions of nucleophilic anions at micelle-water interfaces. In this paper we report observed rate constants (k(obs)) in aqueous micelles of dimeric surfactants. k(obs) in 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1) is constant in a range of concentration, then increases at high [1], suggesting a transition to a different type of aggregate. A H-1 NMR investigation confirms a phase transition at the [1] which corresponds to the reactivity change. (2S,3S)-2,3-Dimethoxy-1,4-bis(N-hexadecy,N-dimethylammonium)- butane dibromide (2) forms very large aggregates and shows a modest catalytic effect. Cyclization of 2-(12-bromododecyloxy)phenoxide ion (PhBr16) in 1 under preparative conditions yields the intramolecular cyclic product only despite a low effective molarity and a relative high substrate concentration This result reveals the presence of a well-organized substrate-surfactant aggregate.