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A methodology based on reoxidation at 298 K with O2(5%)/He pulses followed by quantitative TPO runs allowed us to study the redox state reached by CeO2 and Ln3+ -containing ceria‐based mixed oxide samples Ln3+:La3+ or Y3+) after their reduction in a flow of hydrogen. The temperature range covered in the reduction treatments was from 623 up to 1223 K. During the severe reduction treatments hexagonal sesquioxide phases segregate from all the samples. The reduction temperature at which segregation occurs follows the trend La/Ce < Y/Ce < CeO2. The TPO and H2-TPD traces of reduced samples which showed evidence of segregation in XRD exhibit characteristic features which can also be used as an indication of this effect. The discussion is focussed on the overall and detailed understanding of the reoxidation process and on the analysis of the H2-TPD profiles.