Oxygen-deficient Li2MnO3−x (0 < x < 0.19) have been directly synthesized from lithium-rich manganese oxide, Li2MnO3, using metal hydrides (CaH2 and LiH) as reducing agents at reaction temperatures of 255–265 °C. The structure of the reduced phase determined by X-ray diffraction experiments has oxygen vacancies in the layered structure, causing slightly larger lattice parameters than the stoichiometric Li2MnO3. Reversible oxygen insertion and extraction from the deficient structure observed by the TG measurements suggests fast oxygen diffusion in the layered rock-salt structure. The oxygen-deficient phase shows extremely large 1st charge and discharge capacities of 388 and 305 mAh g−1, respectively.