Double-perovskite oxides, LnBaCo(2)O(5+x) (LnBCO) (Ln=Pr, Nd, Sm, and Gd), are prepared using a solid-state reaction as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performances of LnBCO-Ce0.8Sm0.2O1.9 (SDC) composite cathodes were investigated for IT-SOFCs on La0.9Sr0.1Ga0.8Mg0.2O3-delta (LSGM) electrolyte. The thermal expansion coefficient can be effectively reduced in the case of the composite cathodes. No chemical reactions between LnBCO cathodes and SDC electrolyte, and LnBCO and LSGM are found. The electrochemical performances of LnBCO cathodes and LnBCC-SDC composite cathodes decrease with decreasing Ln(3+) ionic radii, which is closely related to the decrease of the electrical conductivity and fast oxygen diffusion property. The area specific resistances of the LnBCO cathodes and LnBCO-SDC composite cathodes on LSGM electrolyte are all lower than 0.13 Omega cm(2) and 0.15 Omega cm(2) at 700 degrees C, respectively. The maximum power densities of single-cell consisted of LnBCO-SDC composite cathodes, LSGM electrolyte, and Ni-SDC anode achieve 758-608 mW cm(-2) at 800 degrees C with the change from Ln = Pr to Gd, respectively. These results indicate that LnBCO-SDC composite oxides are candidates as a promising cathode material for IT-SOFCs. (C) 2009 Elsevier B.V. All rights reserved.