A complete numerical solution of the Nernst—Planck equations is presented in order to assess the influence of coion transport on particle-diffusion-controlled ion exchange. The theory is applied to the conversion of a H-saturated resin membrane to a Na-saturated membrane when the former is immersed in infinite, well-stirred, 0.1 and 1.0M NaCl solutions. The reverse conversions are also considered. The numerical solution explicitly accounts for the effects of coion transport on the ion-exchange process, and its predictions are consistent with observations already in the literature.