Impedance spectroscopy was used to measure the oxygen reaction kinetics of composite cathodes consisting of La0.8Sr0.2MnO3 (LSM) and Ce0.8Gd0.2O2-x (GDC) on yttria-stabilized zirconia (YSZ) and GDC electrolytes. The LSM-GDC electrodes are analogous to the widely used LSM-YSZ cathodes but have potential advantages since LSM is non-reactive with GDC and the ionic conductivity of GDC is higher than that of YSZ. LSM-GDC cathodes with 0-60 wt.% GDC were characterized over a temperature range of 600-750 degreesC at oxygen partial pressures between 10(-3) and 1 atm. Using YSZ electrolytes, the low-current interfacial resistance was smallest for 50 wt.% GDC, where the value was 0.49 Omega cm(2) at 750 degreesC. This is two to three times lower than that for LSM-YSZ composite cathodes on YSZ with similar structure. The majority of the LSM-GDC cathodes appeared to be limited by adsorption, as well as oxygen ion transfer into the electrolyte.