The electrochemical properties of porous composite cathodes of La0.6Sr0.4CoO3 (LSC) and La0.45Ce0.55O2 (LDC) in anode supported lanthanum-doped ceria (LDC)/lanthanum gallate (LSGM) bilayer electrolyte single cells have been investigated. The composite cathodes with different LDC and LSC contents were in contact with the LSGM layer in the single cells. Comparing with the pure LSC cathode, the interfacial resistance decreased upon the addition of LDC and the optimum content of LDC was 50 wt.%. The variation in ohmic resistance suggests that the composite cathode can suppress Co diffusion from the cathode into the LSGM electrolyte during the firing of the composite cathode onto the electrolyte. The composite cathode with 50 wt.% LDC showed an ohmic resistance near to the calculated resistance of an electrolyte film. For the pure LSC cathode, the optimum firing temperature was about 1150 degreesC, at which both the electrolyte resistance and interface resistance were the smallest. The cathodic interfacial resistance was effectively reduced for the composite cathodes, especially for the cathode with 50 wt.% LDC, which might be due to the suppressing of sintering and the growth of LSC particles from LDC particles during the firing onto the electrolyte. The complicated effects of the composite cathode on the interfacial resistance and ohmic resistance resulted in the best single cell performance at 650 degreesC with a 50 wt.% LDC composite cathode, and the best cell performance above 700 degreesC on the single cell with pure LSC cathode. (C) 2004 Elsevier B.V. All rights reserved.