Partially oxidized zirconium carbonitrides (Zr-CNO) were evaluated as a non-precious-metal cathode used for polymer electrolyte fuel cells. The zirconium-oxide-based catalysts were prepared from zirconium carbonitrides under controlled oxygen partial pressures from 10(-2) to 10(-18) atm at 1000 degrees C. The ORR activity of Zr-CNO showed a volcano-like tendency against the degree of oxidation (DOO). We found a significant lattice-parameter decrease for highly active Zr-CNOs, and this could be ascribed for the formation of oxygen-and zirconium-vacancies in monoclinic ZrO2. On the other hand, carbon originated from starting materials was deposited on the surface of Zr-CNOs during the partial oxidation process. We also found that the oxygen content on the surface of highly active Zr-CNOs was drastically decreased when the amount of carbon significantly decreased during partial oxidation processes. This result indicates that the deposited carbon acted as a strong reduction agency. From XPS and XRD analyzes, there is a linear relationship between the ORR current and amount of oxygen vacancy formed in monoclinic phase. Therefore, we believe that oxygen vacancy formed in monoclinic phase could act as active sites for the ORR. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.079302jes] All rights reserved.