The surface potentials of n-dodecyltrimethylammoniopropanesulfonic acid (DDAPS) micelles in various electrolytes have been evaluated by capillary electrophoresis. This zwitterionic micelle has an inner cationic surface and an outer anionic surface and accommodates anions better than cations, indicating that a negative surface potential is induced by anion-dominated partition. Selectivity terms, i.e., solvation changes of ions and ion association between ions and charged groups in the DDAPS micelles, are introduced into the Poisson-Boltzmann equation for the spherical geometry. This model. allows the interpretation of differences in the ionic partition and surface potential between electrolytes. The selectivity parameters have been determined by assuming agreement between the zeta potential determined by capillary electrophoresis and the calculated outer surface potential of the micelle. The obtained selectivity parameters can also explain the potentiometrically evaluated partition of ClO4- and I-. It has been confirmed that capillary electrophoresis has nide applicability in surface potential, measurements and can detect surface potentials of less than 1 mV. The selectivity origin in the partition into the DDAPS micelles is also discussed on the basis of evaluated parameters. The hydration changes mainly govern the uptake of well-hydrated anions, whereas poorly hydrated anions are partitioned into the micelle principally by ion-pair formation with the cationic groups in the micelles.