Nation ionomer membranes were exchanged with series of pair of cation mixtures between Li+ and other alkali metal cations, and investigated for equilibrium and transport characteristics of ion and water. Samples were prepared by equilibrating the membranes in the mixtures of LiCl and alkali chloride in aqueous solutions of various mixing ratios. The alkali metal cations, especially those of larger atomic number, revealed higher affinity to sulfonic acid groups than Li+, but less water content and higher density of the membrane. It was found that the mobility of cations followed the order of Li+ < Na+ > K+, Rb+ > Cs+. Concentration of sulfonic acid group increased, when Li+ was exchanged with other cations, but ionic mobility of each cation was affected differently by the other cations in the mixed state. The interaction between Li+ and alkali metal cation appeared to be larger for Li/Rb and Li/Cs systems than for Li/Na and Li/K systems, but as long as the ionic transference numbers are concerned, a pair of cations moved independent of each other. The water transference coefficient (electro-osmosis drag coefficient) of Li+ was ca. 15, but decreased to 10 for Na+ and even less when other alkali metal cations existed. In the mixed systems, Li+ and alkali metal cations showed tendency to drag more water molecules than in the individual ions, which indicated that ionic conductivity decreases when cations are coexisting in the membrane. © 2005 Elsevier Ltd. All rights reserved.