The spontaneous formation of vesicles was detected in aqueous mixtures of two cationic amphiphiles: the double-tailed didodecyldimethylammonium bromide, DDAB, and the single-tailed dodecyltrimethylammonium chloride, DTAC. These aggregates appear in a high-dilution region of the system, intermediate between those where monomers and micelles prevail, and are most easily formed at a considerable excess of the single-chained surfactant (DTAC/DDAB molar ratios approximate to 2-20). Vesicles were characterized at 25 degrees C by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light-scattering (DLS) measurements: they present a well-defined contour, are mostly spherical and unilamellar, but show a large size polydispersity, with the more frequent population distributions of diameters from approximate to 40-50 to approximate to 500-600 nm. Apart from intact vesicles (and in most cases coexisting with them), vesicles with ruptured membranes, small bilayer disks (probably discoidal micelles, rarely found), and globular micelles were also visualized by cryo-TEM. Ruptured vesicles and disks were assigned to intermediate structures between intact vesicles and globular micelles. We propose that the main factor which drives the appearance of vesicles in this bicationic system is the difference in the spontaneous curvature (or packing parameter) of the two long-chained surfactant ions.