Transference coefficients, ionic conductivities and equilibrium properties are reported for Nafion(R) 117 membranes containing a mixture of two cations and water. The membranes were equilibrated in 11 kinds of aqueous solutions of HCl and NaCl, with mole fractions of HCl between 0 and 1, all having c(Cl-)= 0.03 kmol m(-3). Some experiments were repeated for Nafion(R) 112 and 115 membranes. By electron probe microanalysis (EPMA), it was found that Nafion(R) 117 membranes have a slightly higher affinity for Na+ than for H+. The water content of the membrane was determined gravimetrically. It decreased from 20.6 +/- 0.1 to 18.4 +/- 0.2 molecules water per cationic site, as the membrane changed from the pure H-form (HM) to the pure Na-form (NaM). The water transference coefficient, t(H2O), obtained by streaming potential measurements, increased in Nafion(R) 117 from 2.5 +/-0.1 to 9.2 +/- 0.1 as the mole fraction of Na+ in the membrane, X-NaM, increased from 0 to 1. Most of the increase occurred for X-NaM > 0.5. The transference number of H+ in the membrane, t(H+)(m), which was determined by an improved emf-method, showed a rapid decrease for X-HM < 0.5. The plot of t(H2O) vs. t(H+) was almost linear, indicating that each ion promotes transfer of a characteristic number of water molecules. The membrane conductivity, measured by an ac impedance method, changed almost linearly with X-HM, With the same results for Nafion(R) 115 and 117. These results imply that possible sodium ion contamination in the fuel cell membranes will promote membrane drying. The significance of the new methodology established in this work for mixed form ion exchange membranes is stressed, with relevance to the membrane contamination problems.