The reversible electrochemical insertion/extraction of Mg2+ and Li+ into/from a crystalline has been found in orthorhombic Mo9Se11 (o-Mo9Se11), the crystal structure of which is composed of molybdenum cluster units. The insertion/extraction reversibility of bivalent ion, Mg2+, could be ascribed to improvement of slow diffusion in the host lattice through the delocalization effect of electrons induced by cluster structure as supposed in Chevrel phase. The characteristic of Li/Mg-ion half-cell with o-Mo9Se11 cathode, such as discharge curve and theoretical capacity, are discussed based on the electronic structure. A part of discharge curve in the insertion process of Li+ was likely ascribed to the psuedogap structure in the density of state for the electronic band. The reversible capacity of o-MgxMo9Se11 was below 40% of the theoretical capacity deduced from the molecular orbital model, whereas over 80% of that was observed in o-LixMo9Se11. The smaller reversible capacity for Mg2+ could be ascribed to the Coulomb repulsion between bivalent Mg-ions confined in the one-dimensional channel of o-Mo9Se11, which highly prevents ion-insertion for bivalent Mg-ions compared with that for monovalent Li-ions. We suggest that both delocalized electronic structure and high dimensional ion-channel are necessary to realize reversible cathodes of Mg-ion battery with high capacity. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.