Abstract Ion-exchange membrane characteristics, such as transport number λ, diffusion coefficient μ, electro-osmotic coefficient φ, and osmotic coefficient ρ, are measured by means of the electrodialytic concentration of sea water. The effects of sea water temperature, electrolyte concentration in desalting cells, and solution velocity in desalting cells on λ, μ, φ and ρ are discussed. The relationships between ρ and λ, μ, φ are expressed by equations. Ion-exchange capacity A R, water content W, ion-exchange group concentration A W, and electric resistance r are measured, and the relations between ρ and A R, W, A W and r are evaluated. Using the relationships between membrane characteristics λ, μ, φ and ρ, electrolyte concentration in concentrating cells C″ of an electrodialyzer is expressed by the functions of ρ. Solubility of electrolytes in concentrating cells C sat″ is estimated from brine evaporation experiments. Saturation current density i sat is defined by setting C”=C sat ″ , and i sat is expressed as a function of ρ. The reliability of the defined i sat is discussed with reference to the electrodialytic concentration of sea water.