A pyrochlore-related Ce2Zr2O8-x phase has been prepared in a reduction reoxidation process from Ce0.5Zr0.5O2 powders. Ce2Zr2O8-x, based on a cubic symmetry with a = 1.053 nm, decomposes in nitrogen at 800 degreesC, but remains stable up to 900 degreesC in air. It shows mixed oxygen ionic and electronic conductivity. The bulk conductivity at 700 degreesC is 4 x 10(-4) S cm(-1) in air and 1 X 10(-2) S cm(-1) in nitrogen, and the activation energy is 1.27 eV in air. In nitrogen, the Arrhenius law is not obeyed, and a curved plot was obtained from 400 to 700 degreesC; then, the conductivity decreased rapidly due to the thermal decomposition of Ce2Zr2O8-x. (C) 2003 Elsevier B.V. All rights reserved.