Liquid-liquid equilibria of systems water (A) + CiEj surfactant (B) + n-alkane (C) have been modeled by a mass-action law model previously developed and so far successfully applied to a series of binary water + CiEj systems and to the ternary system water + C4E1 + n-dodecane. These calculations provide the basis for the presented modeling. The aqueous systems give information about the association constants and the chi(AB)-parameter of the Flory-Huggins theory and the ternary C4E1-system provides universal temperature functions for the chi(AC)-and the chi(BC)-parameter. The three-phase equilibrium for seven ternary CiEj systems (i = 6-12, j = 3-6) has been calculated by fitting one additional parameter for each of both temperature functions to the characteristic "fish-tail" point. The agreement with the experimental data is reasonably well. For systems with very small three-phase areas the results can considerably be improved by individual temperature functions that incorporate the experimental temperature maximum of the "fish" into the parameter fit. Based on the parameters of the system water + C8E4 + n-C8H18 the "fish-shaped" phase diagram of the system water + C8E4+n-C14H30 was predicted reasonably well. (C) 2010 Elsevier B.V. All rights reserved.