The activity of sodium xylenesulfonate in aqueous solution was determined by vapor pressure measurements, the results of which were transformed to mean activities. In addition, surface tension, conductivity, and partial molar volume were determined to obtain complementary information about potential association structures. No sudden change was observed, indicating the association to take place in the concentration range where the increased solubility of lecithin was observed, contrary to the case of surfactant micellization. The solubilization of lecithin in the aqueous solutions of sodium xylenesulfonate showed the association to be cooperative between the two amphiphiles. A model is proposed for the interpretation of the catastrophic incorporation threshold concentration observed in the solubility curve of lecithin. (C) zool Academic Press.