Partially oxidized zirconium, niobium, and tantalum carbonitrides were prepared to discuss a characteristic common to all. The onset potential for the ORR of partially oxidized carbonitrides reached above ca. 0.85 V. The XRD and XPS analyses suggested that both the crystalline structure and the chemical bonding state of the surface of the partially oxidized carbonitrides were very similar to those of the oxides. However, the partially oxidized carbonitrides had lower ionization potential than the oxides. The lower ionization potential indicated that the partially oxidized carbonitrides had some defects on the surface. From these results, the structure of oxides and the highest oxidation state of surface metal with some oxygen defects were essential to have high ORR activity for group 4 and 5 oxide-based compounds. Such oxygen defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites. (C) 2010 Elsevier B.V. All rights reserved.