Partially oxidized TaC0.58N0.42 has been investigated as a cathode for polymer electrolyte fuel cells (PEFCs). Catalytic activity for the oxygen reduction reaction (ORR) significantly depends on the oxidation state of TaC0.58N0.42. TaC0.58N0.42 and Ta2O5 had a poor catalytic activity for the ORR. The onset potential on the partially oxidized TaC0.58N0.42 for the ORR had a maximum value of 0.83 V vs a reversible hydrogen electrode in 0.1 mol dm(-3) H2SO4 at 30 degrees C. X-ray diffraction analysis showed that the specimens, which had a definite catalytic activity for the ORR, had both TaCxNy and Ta2O5 peaks. This result indicated that an appropriate oxidation of the TaC0.58N0.42 was essential to enhance the catalytic activity for the ORR. (c) 2008 The Electrochemical Society.