The aim of this work is to study the reaction synthesis of cobalt chromite in the solid state between tricobalt tetroxide and bichromium trioxide. The diffusing species have been first identified. The experimental curves have been obtained using thermogravimetry under isothermal conditions and various partial pressures of oxygen. A qualitative interpretation of the transformation is proposed, as well as a geometrical model that leads to the variations of the degree of conversion with time.