Li2MnO3 is a popular cathode material in lithium-ion batteries, however in this study Li2MnO3-carbon nanotube composite is utilized as an anode material and its electrochemical reaction mechanism is systematically studied by using electrochemical, synchrotron radiation-based X-ray diffraction and absorption techniques along with the density functional calculations. This material demonstrates a significantly high initial reversible capacity of 1006.4 mAh g−1. Electrochemical characterization suggests that this unique anode material operates by a conversion reaction, involving the formation of Li2O and Mn metal. Diffraction patterns are measured at different state of charge and Pawley fitting of these patterns reveal that during lithiation Li2MnO3 initially converts into Mn3O4 and at the end of discharge, Li2O and Mn phases are formed. During delithiation, Li2MnO3 is not reversibly formed, instead [Li0.5Mn0.5]O-type phase appears after a complete electrochemical cycle. X-ray absorption spectroscopy and density functional calculations are used to confirm the validity of the electrochemical redox reaction mechanism derived from the electrochemical and diffraction measurements.