Nafion (R) conductivity in a proton exchange membrane fuel cell (PEMFC) with the fuel stream containing ammonia is mainly affected by the ammonium ion composition and operating conditions. In this study, the effect of ammonium ion distribution on Nation conductivity was investigated for the first time. The conductivities of two kinds of contaminated membranes having uniform and non-uniform ammonium ion distributions were studied. To simulate a membrane with a well-defined ammonium ion concentration profile, three individual Nation membranes containing known amounts of ammonium ions were physically stacked together. The uniform and non-uniform cases represented membranes having three layers with the same y(NH4+) or step changes in concentration, respectively. Under fuel cell operations, the conductivities of non-uniformly poisoned membranes were ca. 1.07-1.86 times larger than those of uniformly poisoned membranes, depending on humidity, contamination level, and ammonium ion distribution. Consequently, the performance prediction of a cationic-poisoned PEMFC needs to consider any concentration gradients that may exist in MEA. The liquid-phase conductivities of composite membranes were also studied and the results show that conductivity measurements performed in deionized water are not representative of what exists under fuel cell conditions due to rapid redistribution of ions in the Nafion via the liquid phase. (C) 2010 Elsevier B.V. All rights reserved.