Lyotropic phase behaviors of the nonionic and cationic (protonated) forms of dodecyldimethylamine oxide (DDAO) were investigated. Phase diagrams for DDAO + water, hydrochloric acid salt of dodecyldimethylamine oxide (DDAOHCl) + water, and an equimolar mixture of DDAO and DDAOHCl (DDAO(1/2)HCl) + water systems were determined on the basis of polarized light microscopy, small-angle X-ray diffraction, and differential scanning calorimetry. Also, water activity was measured to see the nonideality of water in lyotropic liquid crystalline phases. From the analysis of the composition dependence of the hexagonal unit-cell parameter, the anisometry of the cylindrical micelles in the hexagonal phase, mu (=l/2R(hc), where I and R-hc are length and radius of cylinder, respectively), was estimated. It was confirmed that DDAOHCl and DDAO(1/2)HCl form infinitely long cylinder micelles, whereas DDAO micelles seem short. The characteristics of DDAO(1/2)HCl and DDAOHCl were interpreted in terms of hydrogen bond formation between nonionic and cationic (or two cationic) species. It was suggested that these micelles grow with concentration in the isotropic solution phase domain leading to the transition to hexagonal phase with very long cylindrical micelles.