The activities of surfactant ions (aD) and counterions (aNa) were directly obtained by the EMF measurement of a concentration cell with an ion-exchange membrane and by pNa measurements respectively for aqueous solutions of sodium dodecyl sulfate (NaDS) below and above the CMC. The mean activity and activity coefficient were also calculated. From these data, together with the surface tension (γ) measured by the Wilhelmy method and the amount of adsorption (Γ) measured by the radiotracer method, the following results were obtained: (1) Mean activity was not constant, but increased with the concentration above the CMC. (2) The Gibbs adsorption isotherm was directly verified from the aD and aNa  vs. concentration relations. (3) The decrease in γ observed above the CMC was in agreement with that calculated from the activity data. (4) The logaD  vs. logaNa plots above the CMC showed a linear relation, indicating the validity of the charged phase separation model as the mechanism of micelle formation. The degree of the counterion attachment of the micelles was found to be 0.73 up to about 80 mmol/l. (5) The intermicellar concentrations of Na+ and DS− ions were calculated; the former increases, while the latter decreases with the concentration. (6) The product of the concentration with an osmotic coefficient was linear with the concentration above the CMC, tangent of the slope being about 0.7.

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