To better understand the lithium insertion mechanism on gamma-Fe2O3, we investigated the crystal structure changes of gamma-Fe2O3 in the process of chemical lithium insertion by using the X-ray Rietveld analysis. We newly found that the lithium inserted sample was a mixture of two phases both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium. It was revealed that the chemical lithium insertion process into gamma-Fe2O3 occurs via a two phase reaction, i.e. inserted lithium forms a Li-rich phase in which the quantity of lithium is constant, and the overall lithium concentration is controlled by the mole fraction changes between the Li-rich phase and Li-lean phase. The Li-rich phase is thought to be the same phase previously reported by Bonnet et al. and Pernet et al., in which the iron occupancy of the 16c site was not equalized with that of the 16d site indicating the space group not to be Fm-3m but Fd-3m. Fast lithium insertion could form the two phases, Li-lean and Li-rich, even for an electrochemical lithium insertion process. (C) 2013 Elsevier B.V. All rights reserved.