Activity/correction coefficients of hydroxyoxime extractants and triisooctylamine in extraction systems containing non-solvating diluents were computed from the interfacial tension isotherms. The deviations from the Gibbs isotherm were considered. in which concentration instead of activities were used. The interfacial tension isotherms were matched by the spline function and the polynomial Computed activity/correction coefficients can be used to model the equilibrium of copper and palladium (II) extraction.