Solid state reaction at an interface between doped zirconia and ceria has been analyzed by X-ray diffraction and EDX. The solid state reaction between Y-doped ceria and Y-stabilized zirconia occurred on heating at 1300 degreesC or higher temperatures to form a single cubic fluorite Y-doped ceria-zirconia phase. The reaction proceeded by preferential migration of the Ce component to the zirconia lattice. The reaction significantly lowered the electrical conductivity. The reaction between Sm-doped ceria and Y-stabilized zirconia, on the other hand, proceeded by migration of every component equally, i.e. preferential migration of a specific component was hardly observed. (C) 2000 Elsevier Science B.V. All rights reserved.