Critical micelle concentrations (cmc) of dodecyldimethylamine oxide (DDAO) were determined at 25 +/- 0.05 degrees C as a function of NaCl concentration C-s for both nonionic and cationic species by the surface tension measurements. The critical micelle concentration of the cationic species, cmc(+), was lower than that of the nonionic species, cmc(0), in the range of C-s higher than about 0.2 M, which strongly suggested an attractive interaction between the headgroups of two cationic species in micelles, most probably the hydrogen bond. Log(cmc(0)) decreased linearly with C-s, while log(cmc(+)) gave a nonlinear dependence on the logarithm of the counterion concentration C-g. The nonlinear Corrin-Harkins relation was discussed in terms of the salting-out contribution and/or micelle growth in addition to the contribution of the electric free energy of micelles. Surface excesses of both nonionic and cationic species were very similar and did not depend significantly on C-s up to 3 M NaCl. The surface tensions at surfactant concentrations above cmc, gamma(cmc), decreased linearly either with C-s for the nonionic species or with log C-s for the cationic species. On the basis of these data, surface excesses of Na+ and Cl- were evaluated by the Gibbs adsorption isotherm and compared with those expected from the double-layer theory. The size of the nonionic micelles remained essentially constant over the entire range of C-s examined, while that of the cationics increased with C-s in the range C-s > 0.5 M. At 1 M NaCl, growth of the micelle with the surfactant concentration was observed for the cationics but not for the nonionics.