A chemical model was developed to estimate equilibrium constant for the solvent extraction of Nd and Sm from chloride solution with saponified PC88A. The model could be employed to predict the distribution coefficient of metal from the initial extraction conditions. The model consisted of chemical equilibria, mass balance, charge balance and solvent extraction reaction. By applying the model to the experimental conditions and data, equilibrium constants for the solvent extraction of Nd and Sm were obtained. The effect of the saponification of PC88A on the extraction of Nd was explained in terms of the equilibrium constant obtained in this study and the change of equilibrium pH. The predicted distribution coefficients of Nd and Sm agreed well with those measured. (Received July 14, 2004)