A new thermodynamic model based on the free volume theory and the lattice model was proposed for solid solutions. A significant advantage of the model lies in its ability to predict the thermodynamic properties of solid solutions using only the binary infinite dilute activity coefficients and the predicted values are in good agreement with their experimental data. This shows that the model with a good physical basis is reliable, convenient and economic. (C) 2003 Elsevier B.V. All rights reserved.