Critical micelle concentrations (cmc) of dodecyldimethylamine oxide (DDAO) were determined at 25 +/- 0.05 degrees C as a function of pH or the degree of ionization of micelles alpha(M) in NaCl solutions mainly by the surface tension measurements, supplemented by a unique method applicable to DDAO-break points in the pH-DDAO concentration relationship. Cationic micelles (alpha(M) = 1) showed a smaller cmc than that of nonionic ones (alpha(M) = 0) at 0.2 M NaCl, although the reverse relation was found at 0.1 M NaCl. A minimum of cmc was observed at the values of alpha(M) close to 0.4 and 0.5 for 0.1 and 0.2 M NaCl, respectively. Surface excesses of the solutions in 0.1 and 0.2 M NaCl were in the order : cationics (alpha(M) = I) > nonionics (alpha(M) = 0) > alpha(M) congruent to 0.5. Stability of DDAO micelles was examined on the basis of the cmc data in terms of a simple theory recently developed. The analysis suggested an attractive interaction between the head groups of cationic and nonionic species and also between cationic species, most probably the hydrogen bond, in DDAO mixed micelles. The analysis gave the values of the regular solution approach parameter beta as -2.20, -2.12, and -2.08 for 0.05, 0.1, and 0.2 M NaCl, respectively. Stability analysis was also carried out based on the available hydrogen ion titration data. The two analyses gave slightly but significantly inconsistent results, and the inconsistency indicates the approximate nature of the Gibbs-Duhem relation applied to the mixed micelle pseudophase.