Novel thermodynamically stable vesicle (L-alpha1) and sponge phases (L-3) were investigated using a mixed surfactant system of polyoxyethylene (1.5 mol) myristyl ether sulfate sodium and triethanolamine salt (PMST)/N'-carboxyethyl N'-hydroxyethyl N-aminoethyl dodecylamide [imidazoliumbetain (IB)]/water system with oleic acid (OA) as a cosurfactant. The interfacial tension measured for different IB/PMST weight ratios of the mixed surfactant aqueous solutions has a minimum at a ratio of 7/3 meaning that IB and PMST interact strongly with each other at this ratio. A slightly bluish turbid unilamellar vesicle phase (L-alpha1) was prepared by adding between 1.5% and 1.9% of OA to the 5 wt % mixed surfactant aqueous solution (IB/PMST = 7/3). Measurement of dynamic light scattering and observation using transmission electron microscopy with the freeze-fracture replication method (FF-TEM) indicated that the average diameter of vesicles was around 100 nm. The vesicle phase could be prepared ranging in the concentration of mixed surfactant from 0.2% to 5% at a certain ratio of mixed surfactants and cosurfactant (IB/PMST/OA = 7/3/3.2). As no change in the conductivity and appearance of the vesicle phase was observed after storing for 90 days at 25 degreesC, the vesicle phase is considered thermodynamically stable. By means of phase diagram determination of the IB/PMST/OA/water system in high concentration (>5%) of the mixed surfactant, a clear optically isotropic and low viscous L3 one-phase region was found in a very narrow range of OA/mixed surfactant ratio from 0.82 to 0.83. An increase in the viscosity of this L3 phase was observed after shaking the solution several times by hand. The solution returned to the former state within a few minutes after the shaking. Micrographs of FF-TEM observation clearly showed that the L3 phase changed into a large multilamellar vesicle phase under the shear.