The surface tension of aqueous solutions of dodecylammonium chloride-octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide-OMS mixtures was measured as a function of the total molality of surfactants and the composition of OMS. The results were analyzed according to our thermodynamic procedure; the phase diagrams of adsorption and micelle formation and the diagram illustrating the composition relation between the micelle and the adsorbed film at the critical micelle concentration were drawn. The activity coefficient and excess Gibbs energy in the adsorbed film and the micelle were defined and then evaluated from the phase diagrams. They were found to give useful information on the deviation from ideal mixing and intermolecular interaction in the adsorbed film and micelle.