The effect of ammonia on polymer electrolyte membrane fuel cells (PEMFC) was tested by adding 10ppmNH(3) to the hydrogen feed to PEMFCs based on GORE (TM) PRIMEA (R) membrane electrode assemblies (MEAs). A significant loss in performance was observed. The poisoning process was slow taking 24 h or more to reach a steady state. In some cases no steady state performance was reached during the experiment. The performance loss was reversible in most cases, but only after operation on neat hydrogen for several days. Additions of 1 ppm NH3 for 1 week also resulted in significant performance loss. An MEA based on carbon supported Pt anode and cathode catalyst did not differ from the GORETM MEA based on PtRu anode and Pt cathode catalyst. The performance losses were higher than could be explained by the observed increase in ohmic resistance in the cell. There was also a significant decay in performance in a H-2 vertical bar H-2 cell, especially at high current density where a reaction limiting current was observed showing that the Hydrogen Oxidation Reaction (HOR) was affected. The Oxygen Reduction Reaction (ORR) on the cathode was also significantly affected by ammonia. (c) 2005 Elsevier B.V. All rights reserved.