Abstract Single activities of the decyl sulfate (DeS−), the calcium (Ca2+), and the sodium (Na+) ions in mixed surfactant solutions of varying mole ratios (Rm=Ca(DeS)2/NaDeS) were obtained at 30±1 °C by means of three kinds of EMF cells with ion-exchange membranes or Na+-selective electrode. The results showed that with increasing concentrations above the cmc, the activity of the DeS− decreases gradually, while that of the Na+ increased linearly; the activity of the Ca2+ remained nearly constant in the solution of Rm=2, but it showed a minimum in the solutions of Rm=1, 0.5 and 0.2. From these activities, the counterion attachments of the Ca2+(rCa) and the Na+(rNa) and the degree of micellar charge (1−2rCa−rNa) were obtained. With increasing concentration, rCa diminished gradually and approached a constant value, while rNa and 1−2rCa−rNa changed reversely except in the solution of Rm=2 where they were nearly constant. The Ca2+ was bound preferentially to the micelle surface. The Na+, which almost diffused away for the solution of Rm=2, bound increasingly to lessen the electrical potential of the micelle as Rm decreased. The specific adsorption potentials and the electrical free energies of transfer of Ca2+ and Na+ from solution to micelle were calculated as 1.7 and −18.4 kJ mol−1, and 1.1 and −14.3 kJ mol−1 respectively at their cmcs. It was showed clearly that little dehydration from the counterion occured and the coulombic interaction played an important role in the counterion binding.