The phase sequence of the N-dodecyl-N-octyl-N-methylamine oxide (C(12)C(8)MAO)/HCl/water system with increasing apparent degree of protonation, X, defined as [HCl]/[C(12)C(8)MAO], has been studied. For a 100 mM concentration of C(12)C(8)MAO the following sequence of phases has been observed: L-1/L-2, L-1/L-alpha/L-2, L-1/L-alpha, L-alpha, L-alpha/L-2. The single-phase L-alpha region begins at X = 0.007 and ends at X = 0.35. The upper phase boundary, X*, depends strongly on the acid that is used for the protonation of the surfactant. It is shifted for increasing hydrophilicity of the acid to higher X values. For formic acid X* = 0.95, and for HBr X* = 0.05. A weakly protonated 1% solution of the surfactant is an iridescent L-alpha phase. Both unilamellar vesicles and multilamellar vesicles are observed in cryo transmission electron microscopy and freeze fracture transmission electron microscopy images in the L-alpha phase. The phase sequence with protonation differs from that of single-chain amine oxide surfactants. The synergism between the protonated and the nonprotonated species is very weak in the range X < X*, while the transition from the L-alpha phase to the L-alpha/L-2 two-phase region is considered to be due to synergism. Little or no synergism is observed regarding the surface tension, but synergism does appear in the interfacial tension between decane and the aqueous solution. The viscoelastic properties of the vesicle/L-alpha phase resemble those of densely packed hard spheres. The effects of electric charge on the elastic property of the vesicles could be understood in terms of the osmotic pressure of the solutions. The interlamellar spacing evaluated by small-angle X-ray scattering showed a minimum around X approximate to 0.1, which is interpreted as a result of two opposing contributions. One contribution is the suppression of undulation of bilayer membranes by introduction of electric charges, and the other comes from the increasing total bilayer thickness due to the increasing hydrogen bond formation with increasing X.