The interfacial tension gamma of the ternary two-phase system of water-nonane-ethylene glycol monobutyl ether (C4E1) was measured as a function of the total mole fraction of C4E1 at six temperatures under atmospheric pressure. To analyze the experimental results thermodynamically, it was shown that the gamma value should be plotted not against the total mole fraction of C4E1 but against the equilibrium mole fraction in the nonane and water phases. It was found that the entropy of adsorption of C4E1 from the nonane phase decreases and that from the water phase increases, compared to the entropy of the pure nonane/water interface formation, with increasing surface density of C4E1. Adsorption was discussed on the basis of the thermodynamic quantities of adsorption and transfer of C4E1 molecules.