Abstract <p>Selective electrodialysis with monovalent-ion-selective membranes has shown high efficiency for separating mono- and multivalent ions. However, the separation mechanism is not yet fully understood. This work studies two cation-exchange membranes from Astom, Japan: a standard-grade CSE membrane and a monovalent-cation-selective CIMS membrane. Based on the results of measurements of exchange capacity, water uptake, zeta potential, contact angle, specific electrical conductivity, and diffusion permeability of both membranes, it is suggested that the selective permeability of CIMS towards monovalent cations is provided both by an electrostatic barrier due to the presence of a selective layer with fixed amino groups and by its denser structure and smaller pore size. The latter necessitates partial dehydration of multivalent cations for their access to the pore space. The dependencies of Li⁺ and Mg²⁺ ion flux densities through CIMS on current density were studied during the electrodialytic extraction of Li⁺ ions from a solution simulating the composition of the natural lithium-containing brine of the Angara–Lena basin. The experimental results indirectly confirmed a significant contribution of the dehydration mechanism to the selective transport of Li⁺. Based on the values of the CIMS selective permeability coefficient (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({P_{{\text{L}}{{\text{i}}^ + }{\text{/M}}{{\text{g}}^{2 + }}}}\)</EquationSource> <!--MMTech2560092Troitskiy-m1--> </InlineEquation>), energy consumption, and Li⁺ extraction degree, the range of optimal current densities in the vicinity of half the partial limiting current density of lithium ions was determined.</p>