The electrochemical properties of the interfaces between an SM0.2Ce0.8O1.9 (samaria-doped ceria, SDC) electrolyte and porous composite cathodes consisting of Sm0.5Sr0.5CoO3 (SSC) and SDC have been investigated in anode-supported single cells at low temperatures (400-600 degreesC). The bilayer structures of the SDC electrolyte films (25 mum thick) and the NiO-SDC anode supports were formed by co-pressing and subsequent co-firing at 1350 degreesC for 5 h. The effect of composition, firing temperature, and microstructure of the composite cathodes on the electrochemical properties is systematically studied. Results indicate that the optimum firing temperature is about 950 degreesC, whereas the optimum content of SDC electrolyte in the composite cathodes is about 30 wt.%. It is noted that the addition of the proper amount of SDC to SSC dramatically improved the catalytic properties of the interfaces; reducing the interfacial resistance by more than one order of magnitude compared with an SSC cathode without SDC.