Carbon-supported CoPc-M (where M = Fe, Co, Ni, V as the second active component) nanoparticle catalysts have been synthesized via a combined solvent-impregnation and milling procedure along with the high-temperature treatment. The effects of metal addition and metal loading on the electrocatalytic activity of carbon-supported CoPc were studied. Both heated and unheated catalysts were characterized by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) employing a rotating disk electrode (RDE) in both acid and alkaline solutions to quantitatively obtain the oxygen reduction reaction (ORR) kinetic constants and the reaction mechanisms. The resulting CoPc-Fe/C of 40:60 ratio, heat-treated at 600 degrees C, exhibited promising catalytic performance in terms of the ORR. The optimal loading of Fe was found to be 5 wt% when Fe loading was varied from 1 wt% to 8 wt% nominal. The transferred electron number, n, per oxygen molecule on CoPc-5%Fe/C electrode is in the range of 2.62-2.66 in acid solution and of 3.03-3.54 in alkaline solution compared to pure CoPc/C with the value of n in the range of 2.01-2.14 in acid solution and of 2.7-3.14 in alkaline solution, respectively. XRD, TEM and XPS analysis revealed that dispersion of the particle size of these carbon-supported catalysts is highly improved after modification of 5%Fe, thus the enhanced catalytic activities due to the increase in active centers which might be from the synergistic effect of Co-N-x and Fe-N-x. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.058209jes] All rights reserved.