A mathematical model based on the Nernst-Planck equation has been suggested for predicting the ionic transport rate in a bi-ionic system under the condition controlled by combined film and membrane diffusion. The Na+-H+ system with a cation-exchange membrane was studied experimentally as a test of the model, and the mass transfer coefficients in the liquid film that yielded an optimum fit of the experimental data to the theoretical curves were determined. The obtained mass transfer coefficients were found to be function of the stirring speed and solution composition. The theoretical time course of the concentration calculated using the obtained diffusion coefficients and mass transfer coefficients agreed well with the experimental results.