Tantalum oxide-based compounds were examined as new non-noble cathodes for polymer electrolyte fuel cell. Tantalum carbonitride powder was partially oxidized under a trace amount of oxygen gas at 900 degrees C for 4 or 8 h. Onset potential for oxygen reduction reaction (ORR) of the specimen heat-treated for 8 h was 0.94 V vs. reversible hydrogen electrode in 0.1 mol dm(-3) sulfuric acid at 30 degrees C. The partial oxidation of tantalum carboniride was effective to enhance the catalytic activity for the ORR. The partially oxidized specimen with highest catalytic activity had ca. 5.25 eV of ionization potential, indicating that there was most suitable strength of the interaction of oxygen and tantalum on the catalyst surface. (C) 2009 Elsevier Ltd. All rights reserved.