Partially oxidized NbC0 N-5(0) (5) has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium NbC0 5N0 (5) powder was partially oxidized in N-2 gas containing O-2 of 10(-4) atm at the temperature range of 700-1000 degrees C Partially oxidized NbC0 5N0 5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC0 5N0 5 and completely oxidized Nb2O5 had a poor catalytic activity for ORR The onset potential of the partially oxidized NbC0 5N0 5 for the ORR achieved 0 92 V vs RHE in 0.1 M H2SO4 at 30 degrees C The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites. (C) 2010 Elsevier Ltd All rights reserved