Flame spray pyrolysis, which produces ultrafine particles, was applied to the synthesis of Ce1-xGdxO2-x/2 solid solutions by substituting Gd from a mole fraction of 0-0.40. The solubility limit of Gd in the Ce1-xGdxO2-x/2 solid solution produced by flame spray pyrolysis was between 0.25 and 0.30, which is consistent with the reported value. The as-prepared Ce1-xGdxO2-x/2 particles had a square morphology and a nanometer range in the equivalent diameter. The small particle size made it possible to reduce the sintering temperature of the Ce1-xGdxO2-x/2 solid solution from 1650 degrees C to 1400 degrees C for the ceria-based solid electrolytes produced by the solid state preparation. The maximum ionic conductivity was achieved when the mole fraction of Gd was 0.25. The mole fraction for the highest ionic conductivity was the same as the particles produced by hydrothermal synthesis. However, the ionic conductivity of the Ce1-xGdxO2-x/2 prepared by the flame spray pyrolysis (1.01 X 10(-2) S/cm at 600 degrees C) was higher than that prepared by the hydrothermal synthesis (7.53 x 10(-3) S/cm at 600 degrees C). (c) 2005 Elsevier Ltd. All rights reserved.