A thermodynamic model which considered chemical and phase equilibria as well as mass and charge balance equations simultaneously was developed for the solvent extraction system of CoSO₄NaOH-CH₃COOH-D₂EHPA-Kerosene. Initial and equilibrium concentrations and activity coefficients of various solutes dissolved in the aqueous phase were calculated from the data of total concentrations of salutes. The difference between the activity coefficients of solutes calculated by Edwards equation and those by Debye-Hu¨ckel equation was increased with ionic charge in the experimental ranges. The distribution coefficients of extractant calculated by the model were increased with pH of the aqueous phase and the slope of Log(D_(HA)) vs. pH plot was nearly 1 as expected. The predicted distribution coefficients of cobalt calculated from the model were in good agreement with those obtained experimentally.