ABSTRACTThe surface tension variations of a binary two phases liquid mixture are discussed using the square gradient approximation to the free energy functional. The case of a weakly undersaturated solution at a temperature below the critical point for which we calculate a divergence of the surface tension and the layer thickness is particulary emphasized. Applications of such considerations are proposed to discuss the experimental surface tension isotherm of the liquid Sn-O system we have studied using the sessile drop technic and A.E.S. analysis.