Data are shown to indicate the effect of high NH3 concentrations on the membrane electrode assembly (MEA) performance in proton exchange membrane fuel cells (PEMFCs). Steady-state tests were performed with different NH3 mixtures: 200 ppm NH3 /H-2; 500 ppm NH3/H-2; and 1000 ppm NH3/H-2. Also, transient tests were performed with 200 ppm NH3/H-2 and 1000 ppm NH3/H-2 and data show that poisoning and recovery rates with NH3 are much slower than with CO and that there may be two mechanisms occurring in series during recovery. These slow rates may be exploited to allow for treatments and control schemes that maintain PEMFC performance should the anode stream be exposed to high transient levels of NH3. The significant findings include (i) the ability of a MEA to recover completely in neat H-2 after exposure to 200 ppm NH3 for 10 h (i.e., 5.7 3 x 10(-4) mol exposure); (ii) the performance is lower for 1000 than for 500 ppm at the same dosage (i.e., 5.7 x 10(-4) mol exposure! although the cyclic voltammograms CV of the poisoned electrodes are the same; and (iii) the observation of a two-step process for recovery of the MEA. (C) 2003 The Electrochemical Society.