Spontaneous, single-walled, equilibrium vesicles of controlled size and surface charge can be prepared from aqueous mixtures of simple, commercially available, single-tailed cationic and anionic surfactants. We present detailed phase behavior and structural studies of one such mixture, sodium dodecylbenzenesulfonate (SDBS) and cetyl trimethylammonium tosylate (CTAT) in H2O, as well as results of less complete surveys of other mixtures. The SDBS/CTAT mixture has many features that appear to be common to aqueous mixtures of asymmetric cationic and anionic surfactants. Vesicle formation apparently results from the production of an anion-cation surfactant pair which then acts as a double-tailed zwitterionic surfactant. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time. These catanionic vesicles are stable for periods as long as several years and appear to be the equilibrium form of aggregation.