The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry. Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of

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... s in both CTAB-rich and SOSrich solutions. Bilayer properties of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones. We observe two modes of microstructural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there is macroscopic phase separation. This transition occurs in CTAB-rich solutions and in SOS-rich solutions at higher surfactant concentrations. In the second transition mode, mixtures rich in SOS at low surfactant concentrations exhibit no phase separation. Instead, small micelles abruptly transform into vesicles over a narrow range of surfactant concentration. Since the vesicles that form in mixtures of oppositely charged surfactants are equilibrium microstructures, the microstructural evolution is related solely to the phase transition and is thus under thermodynamic control. This differs from experiments reported on the dissolution of metastable vesicles, such as the detergent solubilization of biological phospholipid membranes, which may be controlled by kinetics. Despite these differences, we find that the evolution in microstructure in our mixtures of oppositely charged surfactants is analogous to that reported for biological membrane solubilization.