Vesicles were found to form easily in diluted aqueous mixtures of two cationic amphiphiles, the double-tailed, bilayer-forming didodecyldimethylammonium bromide (DDAB) and the single-tailed, micelle-forming dodecyltrimethylammonium chloride (DTAC), and therefore are believed to be spontaneous. A combination of Equilibrium tensiometry, conductivity, and static light-scattering techniques enabled the determination of critical aggregation concentrations corresponding to the structural transitions monomers <----> vesicles (CVC) and vesicles <----> micelles (CMC), at several DDAB/DTAC molar ratios, thus delimiting the composition range of the vesicle stability, at 25 degrees C. It was found that the monomers <----> vesicles transition, at lower total surfactant concentrations, is driven by the double-chained DDAB, while the vesicles <----> micelles transition, at higher total surfactant concentrations, is mainly determined by the single-chained DTAC. A region of coexistence of vesicles and micelles was found, whose limiting lower and upper CMC values (CMCL, CMCU) were defined by static light-scattering measurements.