A mathematical model was proposed for a non - steady state film diffusion in impregnating Nafion membrane with dilute platinous solution. Because of the different characteristics of Pt( NH3)42+ and Na+ ions

the diffusion flux is affected by concentration difference and diffusion electric field. The model was solved with a algorithm of predictor - corrector method. Impregnating experiments were carried out to verify the model. The diffusion coefficients of Pt(NH3)42+ and Na+ ions and film thicknesses were determined. Numerical results agreed satisfactorily with experimental data.