Partially oxidized niobium carbonitride has been examined to apply to a non-platinum cathode for polymer electrolyte fuel cells (PEFCs). NbC0.5N0.5 powder was partially oxidized in an inert gas containing 30 or 100 ppm O-2 at 1000 degrees C for 10 h. NbO2 or Nb2O5 was observed in the X-ray diffraction patterns of the specimens. The catalytic activity for the oxygen reduction reaction (ORR) of the specimen that contained Nb2O5 was higher than that of the specimens with NbO2 and as-prepared NbC0.5N0.5. X-ray photoelectron spectra suggested that the highest oxidation state of Nb affected the improvement of the catalytic activity for the ORR.