The compatibility of a composite electrolyte composed of a yttria stabilized zirconia (YSZ) film and a yttria-doped ceria (YDC) substrate in a solid oxide fuel cell (SOFC) that can be operated under 800 C was evaluated. The YSZ film coated on a YDC substrate was derived from a polymeric YSZ sol using a sol-gel spin coating method followed by heat-treatment at 1400 C for 2 h. The SEM and XRD analysis indicated that there were no cracks, pinholes, or byproducts. The composite electrolyte comprising a YSZ film of 2 mum thickness and a YDC substrate of 1.6 mm thickness was used in a single cell performance test. A 0.5 V higher value of open circuit voltage (OCV) was found for the composite electrolyte single cell compared with an uncoated YDC single cell between 700 and 1050 C and confirmed that the YSZ film was an electron blocking layer. The maximum power density of the composite electrolyte single cell at 800 C, 122 mW/cm(2) at 285 mA/cm(2), is comparable with that of a YSZ single cell with the same thickness at 1000 C, namely 144 mW/cm(2) at 330 mA/cm(2). The hypothetical oxygen partial pressure at the interface between the YSZ film and the YDC substrate for the composite electrolyte with the same thickness ratio at 800 C is 5.58 x 10(-18) atm which is two orders of magnitude higher than the equilibrium oxygen partial pressure of Ce2O3/CeO2, 2.5 x 10(-20) aim, at the same temperature.