Critical micelle concentrations (CMCs) of three ionic surfactants that contain n-decyltrimethylammonium ion as a surfactant cation and bromide, perfluoroacetate, and perfluoropropionate anions as a counterion, respectively, were determined by electrical conductivity at several temperatures. The degrees of counterion binding to micelle were obtained at the same temperatures as the change in CMC with total counterion concentration. Molecular weights of micelles were determined by static light scattering. The change in monomeric surfactant ion concentration with total surfactant concentration was determined by membrane potential measurement. The mass-action model was applied to micelle formation to calculate the three micellization parameters : micellization constant, micelle aggregation number, and number of counterions per micelle. Thermodynamic parameters (Delta G(0), Delta H-0, Delta S-0) of the micelle formations were calculated from the temperature dependence of these parameters. At lower temperatures micellization has been found to be entropy driven, whereas it is enthalpy driven at higher temperatures. For n-decyltrimethylammonium perfluoropropionate, however, the tendency is quite different from those of other surfactants. The micellization parameters also made it possible to evaluate the concentration change of monomeric surfactant ion with total surfactant concentration. The evaluated concentration explains the results from the membrane potential measurement.