Phase diagram of iron-cobalt-oxygen has been established experimentally in certain temperature, pressure, and composition ranges (900°C∼1400°C, 10 -4 ∼760 mm Hg, Fe 1- x Co x ∼Oxygen, 0≦ x ≦1/3). In these ragges, when the temperature and the mixing ratio of metallic elements are fixed, it has been found that spinel phase is stable below a certain critical pressure of oxygen. Above this pressure, precipitates of hematite are formed in the spinel matrix, resulting in an abrupt increase of the total oxygen content in the solid phases. The phase boundary was determined by observing the change of the oxygen pressure in a sealed tube, which resulted from the oxidation or reduction of the specimen. Results of several measurements of specific heat on heating from 100°C to 750°C, the variations in oxygen content during heat-treatment, and the change of lattice constants at room temperature with heat-treatment, are presented. Curves of lattice constants vs composition of metallic constituents have a minimum at comp...