Strong electrostatic interactions between the oppositely charged head groups of cationic and anionic surfactants in aqueous mixtures provide an added degree of flexibility in tailoring microstructure. Here, we trace the phase behavior and microstructural evolution in mixtures of surfactants with symmetric tail groups: sodium doidecyl sulfate and dodecyltrimethylammonium bromide. Mixturesof single-chained cationic and anionic surfactants are studied using conductivity, cryo-transmission electron microscopy, video-enhanced microscopy, and time-resolved fluorescence quenching. We find that micelles of anionic surfactants grow upon addition of cationic surfactant. These rodlike micelles are transformed abruptly into vesicles over a very narrow composition range. For this symmetric system, formation of hydrated crystals of 1: 1 anion/cation surfactantdominates thephase behavior. We develop a theoretical thermodynamic cell model to predict important properties of the mixed micellar solutions such as monomer and micellar composition and counterionbinding as well as the equilibria between the crystalline and micellar phases. The cell model provides a good account of both the micelle/precipitate phase equilibria and themicellar solution properties as probed by electrical conductivity.