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The complexity of the ZrO2&-CeO2 phase diagram arises from several factors: the low solubility of each compound into the other one, the slow kinetics of cation diffusion, the occurrence of Ce reduction at high temperatures, and the existence of several metastable phases whose appearance and evolution depend on synthesis method and thermal history of the sample. Identification of phase content is moreover complicated because the X-ray diffractograms of some ZrO2&-CeO2 phases are very close or even indistinguishable, which imposes the use of other techniques more sensitive to small oxygen displacements. In this work we present a Raman spectroscopic study of phase segregation in the ZrO2&-CeO2 system between 1300 and 1650 °C, focusing on the effect of Ce reduction at high temperatures and its relation with the appearance of metastable phases upon cooling. The nature of the high-temperature defective cubic phase is discussed.