Surfactants self-assemble into aggregates above a certain concentration. In this work mixtures of the cationic surfactant cetyltrimetylammonium bromide (CTAB) and the anionic surfactant sodium octyl sulfate (SOS) were investigated systematically. The measurements were accomplished by combining the two complimentary techniques static light scattering (SLS) and cryo-transmission electron microscopy (CRYO-TEM). It was found that CTAB-rich samples contain large threadlike micelles rather close to mole fractions where vesicles start to form. The mole fraction x of the surfactant in excess in the aggregates was calculated and it was found that it differs a lot from the mole fraction in the bulk, and the transition from micelles to vesicles occurs when x is about 0.7. In the SOS-rich samples small globular micelles were observed that transform into vesicles upon dilution. Some of the samples rich in SOS were found to contain open vesicles with CRYO-TEM and the reasons for this behavior have been discussed. One question that needs to be further investigated is whether or not these structures are the result of some kind of distortion of the equilibrium process during sample preparation in connection with CRYO-TEM measurements. In most cases the two methods showed consistent results and trends, but for some samples differences could be observed.