The aim of this work was to investigate the electrochemical performance of yttria-zirconia-titania-ceria ( Y0.15Zr0.57Ti0.13Ce0.15O1.925) and its metal composites in hydrocarbon fuels. The materials were prepared by the ceramic method, and characterized by X-ray diffraction (XRD) thermogravimetry and dilatometry. DC conductivity measurements, at 900 degreesC, were performed as a function of p(O2). The electrochemical performance on its own or as cermets with nickel or copper was investigated using a two-electrode, symmetrical cell format. Experiments were performed in air, 5% H-2 (wet) and 5% CH4 (wet) to investigate the electrochemical processes, utilizing the AC impedance spectroscopy technique in the temperature range 500-900 degreesC. The material is found to retain its cubic fluorite structure on reduction despite all the Ce4+ being reduced to Ce3+. The thermal expansion coefficient is close to that of yttria-stabilized zirconia (YSZ) and the conductivity is largely dominated by oxide ionic, with a small electronic contribution. The copper-based cermets perform well at 600 degreesC, but microstructural degradation due to Cu migration decreases performance at higher temperatures. Surprisingly the electrode performance in air, for CuO/YZTC6, is reasonable. Overall, however, problems with Cu migration indicate that this composite is only an interesting anode for low temperature use (< 600 &DEG;C). Nickel-based cermets did not show such good performance at low temperatures; however, their performance was the best at higher temperatures, but still an order of magnitude lower than Ni/YSZ cermets with optimized microstructure. (C) 2003 The Electrochemical Society.