This review investigates the effects of hydro-organic solvents on ion exchange membranes used in conventional electrodialysis. The thermodynamics of electrodialysis is first presented in relation to operation in purely aqueous solutions, where the Donnan potential describes the equilibrium partitioning at the membrane/solvent interface. The mass transfer kinetics through the membrane are described using the Nernst–Planck equation, and concentration polarization describes the mass transfer resistance in the solution boundary layer. Each of these relationships is found to change significantly as the organic concentration in the solvent is increased and the system consequently deviates from ideality. The extent of membrane swelling in these mixed solvents is also critical, as it determines the diffusion coefficient of both ionic and non-ionic species within the membrane structure.