The mixed micellization between the perfluorooctylbutane trimethylammonium bromide (C8F17) and the cetyltrimethylammonium bromide (CTAB) surfactants in the presence of KBr has been investigated. Surface tension experiments revealed a demixed micelles behavior. At high ionic strength ([KBr] greater than or equal to 100 mM), semidilute solutions exhibit a phase separation between vesicles and micelles when the molar ratio R = [C8F17]/([C8F17] + [CTAB]) exceeds similar to0.3. In the monophasic domain, light scattering and T-jump experiments performed on the dilute regime as well as measurements by dynamic light scattering of the terminal viscoelastic relaxation time in the semidilute range suggest a continuous micellar growth as R increases. At lower ionic strength ([KBr] = 30 mM), the systems remain monophasic in the whole R range. The terminal relaxation time increases continuously with R, the increase becoming very steep in the vicinity of R = 1, which strongly suggests a structural transition within the micellar aggregates.