Ceria and doped ceria nanopowders synthesized in flames from submicrometer-sized droplets of precursor solutions demonstrate several characteristics that promise superior performance when used to fabricate electrolytes for solid oxide fuel cells relative to powders synthesized using other technologies. Combustion chemical vapor condensation has been used to produce CeO2, Ce0.8Sm0.2O2-x (SDC), and Ce0.8Gd0.2O2-x (GDC) nanopowders, with an average particle size below 20 nm. These nanopowders have been used to fabricate electrolyte test specimens. The effect of particle size, dopant, and sintering temperature on electrolyte morphology, density, and ac conductivity have been measured. Sintering of pressed nanopowder specimens at temperatures below 1150degreesC resulted in electrolytes with densities higher than 99% of theoretical density. The resulting sintered grain sizes range from 150 to 500 nm for CeO2 and GDC, and from 150 to 300 nm for SDC. AC impedance measurements show conductivities of 8 and 10 S/m at 800degreesC for SDC and GDC electrolytes, respectively. Conductivities at 500degreesC approached 2 S/m for both SDC and GDC. (C) 2003 The Electrochemical Society.