A new model is proposed for the calculation of activity coefficients. The proposed model is based on the quasi-chemical theory. The surface area parameters in the model depend on partner molecules and concentrations. The surface area parameters for pure components are cited from the surface area parameters of UNIQUAC model. The number of fitting parameters are three for each binary system in the model. These are q(21)(infinity) (surface area parameter of component 1 in component 2 at infinite dilution), q(12)(infinity) (surface area parameter of component 2 in component 1 at infinite dilution), and tau(12) (interaction parameter) for component (1) + component (2) binary system. The activity coefficients of multi-component systems can be calculated with the binary parameters of constituting binary systems. The calculated results by the model are in good agreement with the experimental data of vapor-liquid and liquid-liquid equilibria for binary systems. The ternary liquid-liquid equilibria are calculated by using the binary parameters of constituting binary systems. The calculated results by the model are much better than those by NRTL and UNIQUAC models. (C) 2012 Elsevier B.V. All rights reserved.