The phase separation in 12 mol% CeO2─ZrO2ceramic heattreated in a mixture of H2and Ar was investigated by X‐ray diffractometry (XRD), X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid‐solution phase separated into Zr2Ce2O7and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce4+to Ce3+predominantly occurred, whereas the reduction from Zr4+to Zr3+took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce4+(Zr4+) to Ce3+(Zr3+), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase wa