A partially oxidized has been investigated as a new cathode for polymer electrolyte fuel cells. The catalytic activity for oxygen reduction reaction (ORR) significantly depended on an oxidation state of the . and had a poor catalytic activity for the ORR. The onset potential on the partially oxidized for the ORR had a maximum value of 0.9 V versus reversible hydrogen electrode in at , which indicated that the partially oxidized had definite catalytic activity for the ORR. Hard X-ray photoelectron spectroscopy and X-ray diffraction analyses revealed that the surfaces of the partially oxidized were oxidized, although the inner parts of the partially oxidized remained tantalum carbonitride. These results suggested that an appropriate oxidation of the was essential to enhance the catalytic activity for the ORR.