It was reported that ceria-carbonate composites are promising electrolyte materials for intermediate temperature fuel cells. The conductivity stability of composite electrolyte with co-doped ceria and binary carbonate was measured by AC impedance spectroscopy. At 550 degrees C, the conductivity dropped from 0.26 to 0.21 S cm(-1) in air during the measured 135 h. At a constant current density of 1 A cm(-2), the cell performance keeps decreasing at 550 degrees C, with a maximum power density change from 520 to 300 mW cm(-2). This is due to the increase of both series and electrode polarisation resistances. Obvious morphology change of the electrolyte nearby the cathode/electrolyte interface was observed by SEM. Both XRD and FT-IR investigations indicate that there are some interactions between the doped ceria and carbonates. Thermal analysis indicates that the oxide-carbonate composite is quite stable at 550 degrees C. The durability of this kind of fuel cell is not good during our experiments. A complete solid oxide-carbonate composite would be better choice for a stable fuel cell performance. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.