Three-layer combination formed in dialysis systems, which comprises membrane confined between two diffusion layers, can be considered as a single whole (a fragment) included in numerous complicated electrodialysis assemblies. A mathematical model is developed for estimating the electrodiffusion transfer in such a fragment containing four operating salt ions with the allowance for the membrane heterogeneous structure. The method of solving the problem is based on the integrating of the Nernst-Planck transfer differential equations with the corresponding boundary and interfacial conditions.