This study examines the role of transition metals (Co or Fe) on nitrogen-modified carbon-based catalysts for the oxygen reduction reaction (ORR). The nitrogen-modified carbon-based catalysts are synthesized by the pyrolysis process in the presence of polypyrrole (PPy) and ethylenediamine (ED) with different amounts of transition metals. Electrochemical data and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis do not support that the transition metal itself behaves as an active site for ORR. The X-ray photoelectron spectroscopy (XPS) and elemental analysis results show that the total nitrogen content and the active nitrogen functional groups, such as pyridinic-N and graphitic-N, are strongly dependent on the type of transition metal and the amount of transition metal used. Therefore, it is believed that transition metals serve to catalyze the formation of active nitrogen functional groups for the ORR by doping nitrogen into carbon. (C) 2012 Elsevier B.V. All rights reserved.