The surface tension of an aqueous solution of sodium chloride (NaCl) and dodecylammonium chloride (DAC) mixture was measured as a function of the total molality and composition of mixture at constant temperature under atmospheric pressure. The surface densities of the solutes and the DAC compositions of the adsorbed film and micelle were then calculated. The miscibility of NaCl and DAC in the adsorbed film and micelle was discussed by drawing the composition diagrams of adsorption (CDA) and micelle formation (CDM). Because of the negative value of the surface density of sodium ion due to the electrostatic repulsive interaction with the adsorbed dodecylammonium ions, the surface DAC composition became larger than 1. In a similar way, the micelles also had a DAC composition value larger than 1, probably because of the negative value of the number of sodium ions in the micelles. Furthermore, at the critical micelle concentration, the shapes of the CDM and CDA were similar. It was finally shown that the miscibility of NaCl and DAC in the micelle is similar to that in the adsorbed film and that our thermodynamic approach to elucidate the miscibility in adsorbed films and in micelles by use of the CDA and CDM is also applicable to mixed systems consisting of an inorganic salt and an ionic surfactant.