Ion-exchange equilibria of ammonium between an aqueous phase and Nafion 117 were measured at 10, 25, 40, and 60 degreesC by equilibrating the membrane in 0.1 M chloride electrolytes of known cation composition. The water content in the membrane phase decreased linearly with increasing cation fraction of ammonium in the membrane phase (y(NH4)(+)) from lambda(H2O) = 21.2 (moles of water per mole-sulfonic acid groups) in proton form Nafion in pure water to lambda(H2O) = 13.2 in ammonium form Nafion in a 0.1 M chloride solution. The conductivity was measured by ac impedance in a two-electrode setup using a stack of membranes. The conductivity also decreased linearly with increasing y(NH4)(+) from 97 to 25 mS/cm at 25.0degreesC. Our results indicated that the conductivity of Nafion was isotropic, however, available literature is not conclusive on this matter. The temperature dependence of the conductivity was measured, and the fitted activation energy in an Arrhenius-type equation was found to depend on membrane composition and hence water content. (C) 2004 The Electrochemical Society.