A new thermodynamic model has been developed for solvent extraction equilibria using the K-value method. In this model, the activity coefficients of all solutes in both phases were caluculated by Edwards’ equation. The model provides a means for calculating the equilibrium constant of the extraction reaction from a few experimental measurements. Based on the calculated equilibrium constant, the model can predict the distribution coefficient at any pH, metal concentration and extractant concentration. The calculated values were insensitive to the values of the phase equilibrium constant of the extractant between the aqueous and the organic phases or the formation constant of the extracted metal species and the extractant in the aqueous phase. The calculated distribution coefficients of zinc in the ZnSO4-H2SO4-H2O-D2EHPA-kerosene system were in good agreement with those obtained experimentally. The calculated values for copper in the CuSO4-H2SO4H2O-LIX65N-toluene system and nickel in the NiSO4-CH3COOH-NaOH-Kelex 100-xylene system were also found to be in very good agreement with those obtained from the literature. A similiar procedure is expected to be applicable for other solvent extraction systems involving alkaline solutions or anionic extractants.