Addition of a small amount of cerium (3 and 5 mol.%) remarkably affected the oxidation state of manganese oxide. Manganese oxide without cerium absorbed oxygen in the temperature region from 600 to 900 K to form Mn2O3, while Mn2O3 was not formed in the presence of cerium. In the low temperature region(< 700 K) the valence of Mn was 3.2 in the presence of cerium while it was 2.85 in the absence of cerium. It was concluded that cerium provided oxygen to manganese oxide at low temperature and, on the contrary, withdrew oxygen at high temperatures. This behavior well explained the catalytic action of manganese-cerium composite oxide in the oxidation of CO and in the decomposition of N2O.