A previously published macrohomogeneous mathematical model of the simultaneous transport of multiple ions across an ion-selective membrane under current load based on the Nernst-Planck equation was extended. A significantly more realistic model is proposed and realised. The change with the most significant impact on the results of the model is consideration of convective mass transfer in the external diffusion layers adjacent to the membrane surfaces. This change results in a reduction of the concentration maximum previously observed in the membrane interior and highlights the importance of the external diffusion layers for ion transport across an ion-selective membrane. Hitherto this has often been underestimated.