The phase behavior and microstructural changes in aqueous mixtures of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl tri-oxyethylene sulfate (C₁₂E₃S) are reported. The mixing behavior of the two surfactants is investigated via surface tension measurements and the regular solution approximation yielded a β value of −13, indicating attractive interaction. The difference in the growth behavior of the micelles on the CTAB-rich and C₁₂E₃S-rich sides of the phase diagram is studied using viscosity, quasielastic light scattering, small angle neutron scattering, and cryogenic transmission electron microscopy (cryoTEM). Branched micelles are formed close to the phase boundary and are more prevalent in CTAB-rich micelles. A shear-induced transition of cylindrical to ribbon-like micelles is observed by cryoTEM in CTAB-rich micelles near the phase boundary. A temperature-induced phase transition is observed for C₁₂E₃S-rich micelles due to the dehydration and/or conformational changes of the hydrophilic oxyethylene part of C₁₂E₃S in the mixed micelles.