Ether compounds such as ethylene glycols with different molecular weights and crown ethers have good affinity to the cation exchange membranes, sulfonated styrene-divinylbenzene membrane (sodium ion form) and perfluorocarbon sodium sulfonate membrane. The impregnated amount of ethylene glycols in the membranes was higher than the water content of the membranes. After the ether compounds had been impregnated in the cation exchange membranes, electrodialysis of mixed salt solutions (1:1 mixture of alkaline earth metal ions or potassium ions and sodium ions) was carried out in the presence of the compounds to observe the change in permselectivity between two cations. Though current efficiency did not change in the presence of the compounds, transport numbers of alkaline earth metal ions relative to sodium ions decreased. Namely, sodium ions permeated through the membrane more selectively than alkaline earth metal ions. This is due mainly. to a decrease in the mobility of alkaline earth metal ions in the membrane phase and partially to a decrease in the ion exchange equilibrium constant of alkaline earth metal ions to sodium ions with the membrane. This originates from the difference in ion-dipole interaction between cations and ether groups. The transport number of potassium ions relative to sodium ions also decreased in the presence of the compounds. In particular, the permeation of potassium ions relative to sodium ions remarkably decreased in the presence of 18-crown-6 in the membrane and in the solution due to the formation of a strong complex between potassium ions and 18-crown-6.