Spinel oxides are applied as cathodes (fuel electrode) for the high temperature co-electrolysis of CO2/H2O mixtures. The spinel oxide is composed of two transition metals, which may improve activity for the reduction of CO2 and H2O. A series of spinel oxides have been synthesized and tested in co-electrolysis mode. A good candidate, CuFe2O4, was found and achieved a current density higher than 1 A/cm(2) in CO2/H2O co-electrolysis with a fuel composition of 30%CO2/30%H2O-Ar at 800 degrees C, although the product was enriched in H-2. This material presents superior Coulombic (Faradaic) efficiency for CO + H-2 gas products, reaching more than 95%. It shows stable and reversible I-V curves, indicating good electrochemical cell performance in the co-electrolysis operation. In addition, a uniform porous structure with fine particles was observed after co-electrolysis, which may be associated with the stable and reversible electrochemical cell performance observed.