Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10(-5) atm, CuxCe1-xO2-delta (0 <= x <= 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu0.10Ce0.90O2-delta exhibits the oxide-ion conductivity of ca. 10(-4) Omega(-1) cm(-1) at 600 degrees C at an oxygen partial pressure of 10-5 atm. (c) 2005 Elsevier Ltd. All rights reserved.