The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of the surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurement were analyzed by our thermodynamic procedure. Both the phase diagram of adsorption and th at of micelle formation were found to have an azeotropic point, and the stronger energetic stabilization was suggested. The extent of this nonideal mixing was estimated quantitatively in terms of the excess Gibbs energy, (g) over cap (H,E), in the adsorbed film and of that in the micelle, (g) over cap (M,E). The dependence of (g) over cap (H,E) on the surface tension was also utilized to elucidate the mechanism of the attraction between the different species in these aggregates in detail. By combining the results of the present study with that of the previous one, we concluded that the attractive interaction acting in the adsorbed film and micelle is the indirect interaction between dodecyl sulfate ions (DS-) and the ether oxygen atom of C8E4 molecules through the Na countereations. Sodium ions interact with the DS- ion and C8E4 simultaneously, probably through the electrical attraction.