It was shown that only individual cerium and manganese oxides exist over the subsolidus region of the Ce-Mn-O system in air. This conclusion was based on an X-ray phase analysis of the CeMnOn, Ce0.95Mn0.05Om, and Mn1.95Ce0.05Ok ceramics, which were prepared at 800, 1000, and 1400°C in air and were cooled in various modes. Elements of the diagram of the state for the Ce-Mn-O system were constructed.