The solid-state reaction mechanisms of the CaO-FeO system have been studied by a diffusion couple method in the temperature range of 1073 to 1273 K, under controlled oxygen-partial pressures. The interdiffusivities and intrinsic diffusivities of Ca 2+ and Fe 2+ have been determined in the limited FeO solid-solution range. Based on the obtained results, a solid-state reaction model of the system has been developed, taking account of the formation of the Ca 2 Fe 2 O 5 phase. According to the model, the diffusivities of Fe 3+ in the Ca 2 Fe 2 O 5 phase have been estimated at 1273 K.