The electrodialysis behavior of HI, solution (mixture of iodine and hydriodic acid) was examined using commercial ion exchange membranes, NEOSEPTA CMH and TOSFLEX IE-DF34, from the viewpoint of concentrating HI. The concentration efficiency was found to be governed by ion selectivity and electric resistance of the anion exchange membrane, and also by water permeation through the cation exchange membrane due to osmosis and electroosmosis. The minimum current density required for the concentration was estimated using the measured electrodialysis rate of HI and the water permeation rate.