The formation of the water + ethanol mixture/air surface was studied by measuring the surface tension as a function of temperature and the mole fraction of ethanol in the mixture. The surface composition, the entropy Delta s, and energy Delta u of surface formation were evaluated by adopting the thermodynamic equations. It was shown that the surface was enriched in ethanol molecules compared with the bulk solution. The values of Delta s and Delta u were examined closely by estimating the contribution from the nonideal mixing in the bulk solution and that at the surface to Delta s and Delta u. It was concluded that the surface formation is driven by entropy at lower x(2) and by energy at higher x(2), respectively, and that the surface formation becomes more energetically favorable with rising temperature.