The protonation behavior and the stability of micelles of N-lauroylaminopropyl-N',N'- dimethylamine oxide (C12AmCn; n=3) were studied by the hydrogen ion titration and the equilibrium surface tension measurement. The surface potential of C12AmC3 micelles estimated from the results of hydrogen ion titration was lower than that of dodecyldimethylamine oxide micelles at added NaCl concentration, C(S), of 0.1M. The critical micelle concentration (cmc) estimated from surface tension measurements revealed that a reversal of the stability of the cationic and the nonionic micelles of C12AmC3 takes place in the range of C(S) higher than about 0.2M, that is, the cationic micelles become more stable than the nonionic ones in the range C(S) >or= 0.2M. In addition, the surface tension measurements were carried out for C12AmCn with different spacer length, n, ranging from 2 to 6 methylene units at C(S) = 0.1M. The cmc values of nonionic species were kept at almost constant for n <or= 4, while they decreased with n for n >or= 4. The cmc values of cationic species also tended to decrease with n for n >or= 4. These results were compared with those obtained for alkyldimethylamine oxides, and the effect of amide group and spacer groups introduced into amine oxide surfactants was discussed.