Anode-supported cells comprising Ni + yttria-stabilized zirconia (YSZ) anode, thin (similar to10 mum) YSZ electrolyte, and composite cathodes containing a mixture of La0.8Sr0.2MnO(3-delta) (LSM) and La0.9Sr0.1Ga0.8Mg0.2O(3-lambda) (LSGM) were fabricated. The relative proportions of LSGM and LSM were varied between 30 wt % LSGM + 70 wt % LSM and 70 wt % LSGM + 30 wt % LSM, while the firing temperature was varied between 1000 and 1200degreesC. The cathode interlayer composition had a profound effect on cathode performance at 800degreesC with overpotentials ranging between 60 and 425 mV at 1.0 A/cm(2) and exhibiting a minimum for 50 wt % LSGM + 50 wt % LSM. The cathodic overpotential decreased with increasing firing temperature of the composite interlayer in the range 1000 less than or equal to T less than or equal to 1150degreesC, and then increased dramatically for the interlayer fired at 1200degreesC. The cell with the optimized cathode interlayer of 50 wt % LSM + 50 wt % LSGM fired at 1150degreesC exhibited an area specific cell resistance of 0.18 Ohm cm(2) and a maximum power density of 1.4 W/cm(2) at 800degreesC. Chemical analysis revealed that LSGM reacts with YSZ above 1000degreesC to form the pyrochlore phase, La2Zr2O7. The formation of the pyrochlore phase at the interface between the LSGM/LSM composite cathode and the YSZ electrolyte limits the firing time and temperature of the cathode interlayer.