A study of the transition from spherical micelles to wormlike micelles to vesicles in an amphiphilic system with strong electrostatic interactions is presented. We combine small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM) for a characterization of supramolecular aggregates at low concentration as a function of cosurfactant-to-surfactant ratio. Micellar radii and lengths are monitored, and the phase boundaries are determined. At higher concentration, evidence for an onion-type structure is found.