The effect of contamination by Ca2+ ions in proton conductive membranes for polymer electrolyte fuel cells was investigated systematically. Ion and water transport characteristics of Nafion membranes, which were equilibrated with 0.02 to 0.03 kmol m(-3) of HCl/CaCl2 mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that Ca2+ has much higher affinity than H+ to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site H2O/SO3-, decreased about 19% from 21 for H-form membrane to 17 for Ca-form membrane. The water transference coefficient was obtained from streaming potential measurements of Nafion 115 membranes of various H+/Ca2+ cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca2+ content in the membrane increased, especially when the equivalent fraction of Hi in the cationic exchange sites x(HM) became less than 0.5. Ionic transference numbers for H+ in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H+ became less than 0.5. Membrane conductivity changed linearly with H composition in the membrane. In strong contrast to the interaction mode between H+ and Ca cations during ionic conduction, which appeared almost independent, a certain extent of interference was observed among water molecules as they were carried along by cations in the membrane. It was predicted that if Ca2+ ions enter the fuel-cell membrane, they cause serious effects to membrane drying and result in deterioration of fuel-cell performance. The advantage of this methodology in the study of transport characteristics of fuel-cell membranes is stressed due to ease and accuracy of measurements.