Interfacial reaction between yttria-stabilized zirconia (YSZ) and yttrja-doped ceria (YDC) phases was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis after heating at 1300 degrees C indicated that the solid state reaction between YDC and YSZ accompanied a significant shift of the zirconia lines to lower angle. The lines from the ceria phase were only weakened with the reaction. The reaction completed to give a single cubic fluorite phase of Y-doped zirconia-ceria (YCZ) solid solution after heating at 1300 degrees C or higher temperatures. A Pt marker was inserted at the original interface between YSZ and YDC to observe the process of the solid state reaction. From SEM and EDX analyses, the gradated compositional changes were observed in the zirconia side from the Pt marker after heating at 1700 degrees C. Sintered pellets of YDC and YSZ couple showed Kirkendall-void along the interface of the YDC side because of the preferential migration of the Ce component to the zirconia side.