Unilamellar vesicles were prepared from aqueous mixtures of sodium 10-undecenoate and C(10)NMe(3)Br with molar ratios from 9:1 to 1:9. The existence of vesicles was demonstrated by negative-staining TEM, freeze-fracture TEM, bromphenol blue entrapment, and quasi-elastic light-scattering experiments. In general, the diameters of the vesicles ranged from 200 to 1000 Angstrom. The vesicles formed from the nearly equimolar ratio solutions have a larger mean size. The bilayer structure of a vesicle was observed by negative-staining TEM. The vesicles showed two phase transition temperatures, which varied with the sodium 10-undecenoate : C(10)NMe(3)Br molar ratio. The highest temperatures corresponded to the samples with 1:1 ratios. The vesicles remained stable for at least 70 days at room temperature and in at least 25% (v/v) ethanol solution. It is suggested that the strong interaction between the cationic and anionic ions is a prerequisite for the formation of the vesicles. The composition of vesicles varied with the sodium 10-undecenoate:C(10)NMe(3)Br ratio in solutions, and micelles and vesicles existed simultaneously.