The surface tension of aqueous solutions of dodecylammonium chloride (DAC) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurements were analyzed by our thermodynamic procedure, and phase diagrams of adsorption and micelle formation were drawn. The miscibility of the surfactants and their intermolecular interaction in both the adsorbed film and the micelles were examined. Deviation from ideal mixing was observed and an attractive interaction was suggested. Furthermore, the phase diagram of micelle formation was shown to have an azeotropic point, and a stronger energetic stabilization was suggested in the mixed micelle formation than in the adsorbed film at the cmc. This difference suggests that the structures of both the surfactant molecules and the aggregates significantly affect the intermolecular interaction between surfactants.