Ion and water transport characteristics of perfluorosulfonated ionomer membranes are investigated in the mixed cation form of H/Fe, H/Ni, and H/Cu systems: Nafion membranes, which were equilibrated with HCl/FeCl3, HCl/NiCl2, or HCl/CuCl2 mixed aqueous solutions of various mixing ratios, were prepared as test samples, and equilibrium and transport properties were measured systematically. Membrane cationic composition showed that trivalent cations had more affinity than divalent cations. Also larger valence cations caused less water content in the membrane. The membrane ionic conductivity was markedly influenced by counterions, and H+ mobility mu(H)(+) was altered according to the nature of coexisting cations. In the presence of CU2+, mu(H)(+) increased from its inherent value, while in the presence of Fe3+, mu(H)(+) decreased to a large extent, Ni2+ bringing about nearly no change in mu(H)(+). The ionic transference number of H+ was also influenced by coexisting cations in several ways. Despite thp unique influence of impurity cations on rha mobility of H+, the mobility of impurity cations was not affected by the presence of H+. The interaction between adjacent cationic species in the membrane ion exchange sites, although plausible in general for multivalence cations, appeared to be not specific due probably to the shielding of the cationic charge by water molecules or by sulfonic acid groups. The water transference coefficient tH(2)O as measured by streaming potential measurements showed unique changes with membrane ionic composition, and tH(2)O increased from 2.5 to over 13 by the presence of impurity ions. These impurity ions were found to result in more water molecules dragged than in the case of individual ions, when coexisting with the H+ ion. Overall, it was noted that the water molecules within the influence of impurity cations appeared to play a large role in the H+ movement in the membrane.