The ammonia-treated carbon-supported cobalt tungsten (Co-W/C) was prepared by a reaction employing a temperature program in a stream of NH3 between 773 and 1073 K. The effects of the NH3 heat-treatment temperature, Co-W ratio and the preparation method were investigated. The activity of Co-W/C for the oxygen reduction reaction (ORR) was evaluated using a rotating disc electrode and single fuel cell measurements. The Co-W/C prepared by the impregnation method with the same atomic ratio of Co and W, and NH3 heat treated at 823 K, exhibited the highest ORR activity with an onset potential of 0.74 V (vs. RHE at 5 mu A cm(-2)). The XRD and temperature-programmed measurements revealed that the catalyst active species were ascribed to the presence of CoW oxynitride and Co nitride. The catalyst surface was characterized as nitrided metal (accommodation of the N-atom in the host Co-W lattice resulting from a sufficient distance between the Co-W atoms and N) and N-containing carbon (Co surrounded by N-atoms and attached to the C support) by X-ray photoelectron spectroscopy. The Co-W oxynitrides, Co nitride and pyrrolic-type nitrogen of the N-containing carbons are likely responsible for the good ORR activity. (C) 2009 Elsevier Ltd. All rights reserved.