An understanding of high-temperature aging effects on the electrical properties of electrolytes is very important in selecting optimum compositions for practical applications. The aging behavior and mechanisms of doped zirconia ceramics have been extensively studied. However, little information is available regarding the aging behavior of ceria-based electrolytes. The present study has demonstrated that a high-temperature aging at 1000 degreesC has a significant effect on the ionic conductivity of the Y- or Gd-doped ceria (Ce1 - xYxO2 (- delta) and Ce1 - x GdxO2 - delta), especially in the case of the Gd doping. The aging behavior is characterized by a critical dopant concentration, above which the aging has a detrimental effect on the conductivity of the doped ceria ceramics. The aging behavior in the doped ceria cannot be explained using the aging mechanisms applied to the doped zirconia. Instead, the formation of the microdomains in the doped ceria has been acknowledged to be the main contribution to the aging behavior of the Y- or Gd-doped ceria ceramics. The formation ability of microdomains has been estimated to be in the order of La3+>Gd3+>Y3+, based on the degree of size mismatch between the dopant ion and Ce4+ ion. The critical dopant concentrations at which the microdomains start to form for La3+, Gd3+ and Y3+ in the doped ceria ceramics are x = similar to 0.15, similar to 0.2 and similar to 0.25, respectively. This critical dopant concentration is also an important indication: below which the conductivity is governed by only the association enthalpy, and above which the conductivity is dominated mainly by the microdomains rather than the association enthalpy. (C) 2004 Elsevier B.V. All rights reserved.