This study is focused on the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of YBa1-xSrxCo2O5+delta (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The high oxygen nonstoichiometry, delta = 0.18 -0.43 at 700 degrees C, indicates the large oxygen vacancy concentrations in oxides. The electrical conductivity is improved due to the greater amount of electronic holes originated from the increased interstitial oxygen, and the conductivities of all samples are above 100 S cm(-1) at 400-700 degrees C in air. The results demonstrate the promising performance of YBa1-xSrxCo2O5+delta cathodes at intermediate temperatures, as evidenced by low area-specific resistances (ASRs) e.g. 0.21-0.59 Omega cm(2) at 700 degrees C. The lowest ASR, 0.44 Omega cm(2), and the cathodic over-potential, -40 mV at a current density of -136 mA cm(-2), are obtained in YBaCO2O5+delta cathode at 650 degrees C. The dependence of polarization resistance on oxygen partial pressure suggests that the charge transfer process is the rate-limiting step for oxygen reduction reaction in YBaCO2O5+delta cathode. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.