Mixtures of oppositely charged surfactant in aqueous system self assemble to form to a variety of microstructures such as micelles, vesicles, lamellar sheets and nano discs. In this study, various microstructures arising in the water rich corner of CTAB/SDS/Water system with excess anionic surfactant is evaluated with Small Angle X-ray Scattering (SAXS) and the partial phase diagram is obtained. Unique microstructural transformation in the presence of excess anionic surfactant at the dilute region led to prolate micelles first transforming to spherical micelles and then to rod-like micelles. The strong electrostatic interaction between ions in these mixed surfactant aqueous solutions leads to phase separation with multitude of microstructures. The size and shape of a self–assembled aggregate is the product of a balance between different free energy contributions. In order to minimize the electrostatic repulsions between the polar head groups pure SDS self-assemble to form prolate micelles. Upon addition of the cationic surfactant CTAB to SDS, electrostatic attraction circumvents repulsion between like charges of the surfactant, and elongated microstructures evolve by counteracting the initially formed anisotropic structure. The growth and transition of micelles from prolate to rod-like is evident from SAXS scattering curves. Also, spontaneous formation of vesicles is observed at a locus of concentrations depending on mole fractions of the surfactant mixture and the overall concentration. Light scattering confirms that vesicles are poly disperse with average size of 200 nm. Transition from micelle to vesicle seems to be first order from the study.