Abstract: A semi- empirical correlating and predicting system for activity coefficients is proposed. The activity coefficients of binary systems are predicted by an excess Gibbs energy model in which the model parameters are evaluated from two algebraic equations which relate limiting activity coefficient and (dr/dx)x=0 to the model parameters. The Limiting activity coefficient is correlated with the MOSCED model, and (dr/dx)x=0 from Hansens three -dimensional solubility parameters. Predictions for 145 isothermal polar binary systems are compared with experimental values.