Highly stable carbon-supported hafnium oxynitride (HfOxNy-C) was synthesized by heating carbon-supported hafnium oxide, prepared using an impregnation method, under NH3 gas in various conditions. X-ray diffraction patterns, X-ray photoelectron spectra, and field-emission transmission electron microscope images confirmed that HfOxNy nanoparticles were dispersed onto commercial carbon black, Vulcan XC-72. The stability of HfOxNy-C in 0.1 mol dm(-3) H2SO4 at 303 K was evaluated by measuring the mass ratio of dissolved hafnium to immersed HfOxNy-C using inductively coupled plasma atomic emission spectroscopy. It saturated at a low level of 0.8-4.0 mg g(-1) with increasing immersion time up to similar to 24 h. The oxygen reduction reaction (ORR) activity and rate were evaluated by obtaining cyclic voltammograms and rotating disk electrode voltammograms, respectively. The HfOxNy-C exhibited higher ORR activity and a lower Tafel slope than NH3-treated C under identical conditions, demonstrating that HfOxNy is active toward ORR. The ORR activity most depended on the heating temperature. The ORR rate increased with increasing the heating time at 1223 K which could be due to the increased y in HfOxNy-C. The maximum onset potential for ORR was 0.78 V vs. standard hydrogen electrode, which is 0.18 V lower than that of carbon-supported platinum. (c) 2011 Elsevier Ltd. All rights reserved.