Partial molar thermodynamic quantities for oxygen in nonstoichiometric cerium oxides were determined by thermogravimetric analysis in CO/CO2 mixtures in the temperature range 900–1400°C. Under these conditions compositions within the range 2.00 greater-or-equal, slanted O/M greater-or-equal, slanted not, vert, similar1.75 could be obtained. A detailed analysis of the data shows that the α′-phase region in the phase diagram, previously described as a grossly nonstoichiometric phase, can be divided into several subregions each consisting of an apparent nonstoichiometric single phase. The finer details of the thermodynamic data, however, suggest that some of these subregions can be further split into ordered intermediate phases with compositions following the series MnO2n−2. Supplementary high-temperature X-ray diffraction studies under vacuum were made at temperatures up to 855°C. At the higher temperatures between 790 and 855°C, a new phase of low symmetry was obtained. Indexing of the powder pattern for this phase showed it to be isostructural with Pr6O11 and with a monoclinic unit cell with a = 6.781 ± 0.006Å, b = 11.893 ± 0.009 Å, c = 15.823 ± 0.015 Å, and β = 125.04 ± 0.04°.