The asymptotic approach of theory to real processes is characteristic of the modern relationshipbetween the theory and the practice of electrodialysis (steady-state regime, smooth channels, low Reynoldsnumbers etc.). However, at this theoretical level, even the most widely used direct-flow electrodialysis is presented in the literature with solutions that are insufficiently general and, as a rule, hardly usable. In this workthe generalization of classical Sonin–Probstain problem, its analytical solution, and the resulting practical consequences have been investigated, and explicit asymptotic expressions for the electrolyte solution concentrations at the outlet of deionizing and concentrating compartments have been derived for electrodialyzers withsufficiently long channels. The proposed theory is applicable for electrodialysis with ion-exchange membranesof real selectivity and various distances between the membranes, hydrodynamic regimes, and input concentrations in adjacent compartments. The theory makes possible the express calculation of output concentrationsfrom the voltage imposed on the electrodes of multicompartment apparatuses, as well as, generally, from thecurrent between the electrodes.