The relationship between electrical conduction and dielectric relaxation was investigated for 20 at.% Sm doped CeO2 (Ce0.8Sm0.2O2-delta), which is a typical oxide-ion conductor. Numerical calculation clarified that the anomalously large dielectric constant (Et) originated from the superimposition of both Debye-type polarization and interfacial polarization between electrolyte and electrode. Two kinds of the Debye-type relaxation appeared at and above 673 K, which were assigned to defect associates, (Sm-Ce -Vo)center dot and (Sm-Ce')Vo-Sm-Ce)(x). The Debye-type polarization was also confirmed by analyzing the dielectric loss factor (epsilon(r)). Ac conductivity (sigma(ac)) in high temperature and high frequency regions agreed with do conductivity (sigma(dc)), while the dispersion of aa, was ascribed to the Debye-type polarizations. 2007 Elsevier B.V. All rights reserved.