Investigations on the MnO2-Fe2O3 system roasted in air has been reported in our previous work. This study further investigated the MnO2-Fe2O3 system roasted under CO-CO2 atmosphere. Extensive investigations were concentrated on the reduction of simplex iron oxides or manganese oxides, and little attention were paid on the reduction of MnO2-Fe2O3 system regarding to interactive reactions between them. In this work, it was found that spinel-type MnxFe3-xO4 with high magnetism formed easily under CO-CO2 atmosphere. The reduction and thermodynamic analyses of pure MnFe2O4 were also researched to better understand the reduction behaviors of MnO2-Fe2O3 system. Phase study showed that a series of Mn-Fe composite oxides, including MnxFe3-xO4 and (MnO)(y)(FeO)(1-y), generated during the reduction of MnO2-Fe2O3 system. MnxFe3-xO4 was readily generated under CO content of 2.5-25 vol% at 1000 degrees C. With further increase of CO content, MnxFe3-xO4 was reduced to (MnO)(y)(FeO)(1-y) and then to MnO and metallic iron. Reduction of manganese oxides, iron oxides and manganese ferrites happened concurrently during the reduction of MnO2-Fe2O3 system. And the reduction of MnO2, Fe2O3, Fe3O4 and MnFe2O4 were compared by TG and thermodynamic analyses. In addition, the morphology evolution and magnetism change of the MnO2-Fe2O3 system reduced under different CO contents were also studied. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.