The effect of NH3 and NH4+ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH4+ compositions were measured in deionized water at room temperature and compared to those at 80 degrees C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH4+ composition in the membrane (y(NH4+)), with that of the NH4+-form having a conductivity 25% that of the H+-form. The gas-phase conductivity of the NH4+-form, on the other hand, declined by 66-98% relative to the H+-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH3 at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH3 concentration, but the time to reach steady-state varied with NH3 concentration. The y(NH4+) at steady-state conductivity was equivalent for all the NH3 concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and y(NH4+) appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling. (C) 2009 Elsevier B.V. All rights reserved.