Catalysts for the oxygen reduction reaction in polymer electrolyte membrane (PEM) fuel cells were prepared by impregnating four high surface area activated carbons (ACs), namely, Ax-21, CNS-201, IRH-33, and IRH-40, with 0.2 wt % nominal Fe as ferrous acetate, and then by pyrolyzing them for various times in NH3 at 950 degrees C. Among the catalysts made with as-received ACs, high activity was only obtained for those made with Ax-21, which is the only AC having mainly carboxylate surface groups. In the aerated aqueous solution of iron(II) acetate (pH 5 +/- 0.5) used for Fe impregnation, these surface functionalities ligated with free Fe2+ ions and subsequently yielded active sites upon pyrolysis in NH3. It is, nonetheless, possible to obtain high activities using other ACs provided that they are first treated in concentrated HNO3 to generate carboxylic functionalities at their surface. It is believed that Fe/N/C-type catalytic sites are hosted in AC micropores. Fuel cell tests performed with the best catalyst from each AC series show very similar polarization curves with an initial mass activity at 0.8 V ranging from 11 to 22 mA mg(-1). (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3242293] All rights reserved.