Brownmillerite oxide Ca(2)Fe(2-x)Co(x)O(5) (x = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca(2)Fe(2-x)Co(x)O(5) has no reaction with Sm(0.2)Ce(0.8)O(1.9) (SDC) electrolyte at 1100 degrees C for 10 h in air. The thermal expansion coefficient (TEC) of Ca(2)Fe(2-x)Co(x)O(5) increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca(2)Fe(2-x)Co(x)O(5) were studied under various temperatures and oxygen partial pressures. The polarization resistance (R(p)) of Ca(2)Fe(2-x)Co(x)O(5) with x = 0.2, 0.4 and 0.6 are 0.23, 0.48 and 1.05 Omega cm(2) at 700 degrees C in air, respectively. The rate-limiting step for oxygen reduction reaction was the charge transfer process. Ca(2)Fe(1.8)Co(0.2)O(5) cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm(-2) at 700 degrees C in air. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.