The oxygen reduction reaction mechanism of NdBaCo2O5+delta cathode for intermediate-temperature solid oxide fuel cells was investigated by the electrochemical impedance spectroscopy under cathodic polarization. The Nyquist diagrams showed the different changes with the applied cathodic voltage at three temperature ranges: at 500 and 550 degrees C, at 600 degrees C, at 650 and 700 degrees C, which might be related to the changes in the charge transfer and/or oxygen diffusion processes including O-2 adsorption/desorption. Besides, the diffusion process was more easily affected by the increase of applied cathodic voltages than the charge transfer process, which was ascribed to the low activation energy of the diffusion process. (c) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.