Ion and water transport characteristics of Nafion ionomer membranes were investigated systematically in the mixed cation form of H+ and various kinds of alkali metal cation systems, which were prepared by equilibrating C the membranes in the mixtures of HCl and alkali chloride in aqueous solutions of various mixing ratios. The membrane cationic composition showed that cations of larger atomic number had a higher affinity to sulfonic acid groups but less water content in the membrane than those of smaller atomic numbers. The net ionic conductivity was decreased, in any case, by the presence of alkali metal cations in the membrane. Different kinds of the interaction mode among cations were observed between H/Li or H/Na systems and H/K, H/Rb, or H/Cs systems. The interaction between alkali metal cations appeared to increase as the atomic number of the alkali metal cation increased. The water transference coefficient (electro-osmosis drag coefficient) increased from 2.5 to more than 10 by the presence of alkali metal cations. In the mixed systems, these cations were found to cause less water molecule drag than in the case of individual ions, in the presence of H+ ion. Overall, the transport characteristics of H+ and alkali metal cations influenced each other by way of the water molecules when they coexist in the membrane.