The surface tension gamma of aqueous solutions of tetraethyleneglycol monomethyl ether (C1E4) and dodecylammonium chloride (DAC) mixture was measured as a function of the total molality m and the composition X-2 of the mixture at 298.15 K under atmospheric pressure. It was found that the two break points exist on the gamma vs m curves in the very low concentration region and at the critical micelle concentration except for the low composition region. It was concluded that the former break point corresponds to the phase transition of the adsorbed film from the gaseous to the expanded state. The excess Gibbs energy and the activity coefficient of adsorption were calculated to examine the interaction between C1E4 and DAC molecules in the adsorbed film quantitatively. The results showed that the interaction between C1E4 and DAC molecules is favorable in the gaseous state and unfavorable in the expanded state. This finding shows that the drastic change of the miscibility in the adsorbed film is induced by the phase transition. Furthermore it was seen that the C1E4 molecules are miscible with DAC molecules in the micelle rather than in the adsorbed film. This may be due to differences in geometry between the micelle and the adsorbed film.