Critical micelle concentrations (cmc) of dodecyldimethylamine oxide(DDAO) were determined at 25 degrees C in solutions in the absence of salt at different pH values by four methods : surface tension, dye solubilization (Sudan III), pH-concentration break points, and counterion (Cl-) activity. With the aid of the cmc values, the degree of ionization of DDAO micelle alpha(M) was determined as a function of pH from the hydrogen ion titration. The cmc increased sharply in a narrow range of alpha(M) from 0.35 to 0.40. This destabilization of micelles was also noted on the anomalous change of pK on the hydrogen ion titration curve. The counterion activity was measured potentiometrically, and the degree of the counterion binding theta was determined as a function of alpha(M). The value of a showed a sharp increase with alpha(M) also around 0.4. The line width of NMR absorption peak of Cl-35 ions was well correlated with alpha(M). The surface electric potentials psi(0) of DDAO micelles were evaluated from both the hydrogen ion titration and the acid-base indicators, and the former value was larger than the latter one. A procedure is proposed to compare the stabilities of the micelles having different alpha(M) values. The result indicates much larger dependences on alpha(M) than those uncorrected for different ionic strengths at the cmc.