The theory of coupled diffusion of multiple ionic species based on complete Donnan exclusion has been applied to a diffusion process involving an ion-exchange membrane and three counter ions. The influence of the main process parameters (e.g., diffusion coefficients and ionic charges) has been studied by numerical simulations. The results obtained underline the importance of taking into account the coupled diffusion of all counter ions in a multi-ion system to describe adequately the transport of each individual counter ion.