The effect of the atmosphere of heat-treatment on the valence and the coordination state of the cobalt ions were investigated. Cobalt carbonate and α-alumina were mixed in the equimolar ratio, and the mixture was heat-treated at 1400°C or 1300°C under various partial pressure of oxygen (Po2) from 2.1×10-1 atm to 1.7×10-7 atm for the heat-treatment at 1400°C, and from 2.1×10-1 atm to 1.1×10-8 atm at 1300°C. The lattice constants and the photoacoustic spectra (PAS) of the samples were measured. The lattice constants were scarcely changed with temperature and oxygen partial pressure of the heat-treatment (Table 1). In the PAS of the samples heat-treated in low Po2 region, only the peak corresponding to the 4A2(F)→4T1(P) transition of Co2+ ions in the tetrahedral sites was observed. On the samples heat-treated in high Po2 region, however, additional peaks were observed, correspoding to the 4T1g(F)→4T1g(P) transition of Co2+ ions in the octahedral sites and also to the 1A1g(D)→1T2g(D) and 1A1g(D)→1T1g(D) transitions of Co3+ ions in the octahedral sites (Figs. 1 and 2, and Tables 2, 3 and 4). Then the following conclusions were deduced. In low Po2 region, the cobalt ions in the samples were all bivalent and occupied the tetrahedral sites (normal spinel). In high Po2 region, the following four reactions were supposed to take place at the same time. (1) The Co3+ ions were formed by the oxidation of the Co2+ ions. (2) By the reaction (1), the quantity of the Co2+ ions was decreased. Then the quantity of Al2O3 exceeded the stoichiometric quantity of CoO, and the defective spinel was formed. (3) The Co3+ ions substituted for the Al3+ ions in the defective spinel. (4) A part of the Co2+ ions occupied the octahedral sites. The lattice constants were favored to decrease by the formation of the defective spinel (reaction (2)), but favored to increase by the substitution of the Co3+ ions for the Al3+ ions (reaction (3)). As the result of the competition of the above two reactions, the lattice constants were scarcely changed.