Double-layered perovskite oxides of Sm1-xBaCo2O5+delta (S1-xBCO) with various A-site Sm3+-deficiencies (x = 0.00-0.08) were synthesized and evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Sm3+-deficiency content in S1-xBCO was limited up to x = 0.05, and higher content x = 0.08 caused impurity phase. S1-xBCO oxides were chemically stable with GDC electrolyte at 1050 degrees C and below. Introduction of Sm3+-deficiency caused decreased oxygen content and increased concentration of oxygen vacancy in S1-xBCO. Electrical conductivities of S1-xBCO decreased with increasing temperature in air, and also changed with the Sm3+-deficiency content. Electrochemical performance of S1-xBCO cathodes were characterized by impedance spectra measurement based on symmetric cells. Higher Sm3+ deficiency content has resulted in decreased area specific resistances (ASRs) and activation energy (Ea), i.e. enhanced electrochemical reaction reactivity for the S1-xBCO cathodes. Among the studied samples, the S0.95BCO (x = 0.05) oxide showed the best electrochemical performance with ASR values of 0.316 Omega cm(2) at 600 degrees C, 0.137 Omega cm(2) at 650 degrees C, 0.068 Omega cm(2) at 700 degrees C and 0.038 Omega cm(2) at 750 degrees C respectively, thus it's a promising cathode material of IT-SOFCs. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.