The influence of a small amount of cetylpyridinium chloride on the phase diagram of the diluted C12E4/benzyl alcohol/water system is studied. The presence of the ionic surfactant results in the stabilization of the vesicles in relation to the open bilayers, The bilayer structures are less sensitive to shear effects than for the system without cationic surfactant, Moreover, the formation of the mixed micelle phase is also favored. The transition from spherical to rod micelles has been detected from viscosity measurements, The micelles located close to the two-phase region (at the lower amount of cationic surfactant) are the largest and most flexible micelles.