Dielectric relaxations were investigated for the solid solution system Cc I Nd-1-x(x),O2-delta (0.0 <= x <= 0.5) having a fluorite-type structure, which is a typical oxide-ion conductor. The dielectric constants showed anomalously large values at low frequencies and high temperatures. Numerical analysis of frequency dependence of dielecti c constant (epsilon(')(r)) clarified that the anomalously large epsilon(')(r) originated from the superimposition of both Debye-type polarization and interfacial polarization between electrolyte and electrode. Two kinds of Debye-type relaxations observed were ascribed to defect associates, (Nd-Ce' - V-O'') and (Nd-Ce', - V-O-Nd-Ce' )(x). The Debye-type polarizations were also confirmed by analyzing the dielectric loss factor (c,epsilon(')(r)). When the oxide-ion conductivity decreased in the heavy Nd-doped samples, both the Debye-type and the interfacial polarizations also decreased, suggesting that ordering of oxygen vacancy suppressed the electric field response of Debye-type polarization, resulting in the decrease of oxide-ion conductivity. (c) 2007 Elsevier B.V.. All rights reserved.