Li2MnO3 is shown to be electrochemically active, with a maximum charge capacity of similar to 350 mAh/g and a discharge capacity of similar to 260 mAh/g at 25 degrees C. A total of 1 mole of Li can be extracted from Li[Li1/3Mn2/3]O-2, and the first cycle efficiency is similar to 66% regardless of state of charge. Larger charge-discharge capacity is obtained from materials with smaller particle size and larger amount of stacking faults. Composition and structural analyses indicate that Li are removed from both the Li and transitional metal layers of the material during charging. Results from X-ray-absorption fine-structure measurements suggest that the valence of Mn remains at 4+ during charging but is reduced during discharging. Charging is accompanied by gas generation: at 25 degrees C, oxygen is the main gas detected, and the total amount accounts for similar to 1/8 mole of O-2 generation from Li[Li1/3Mn2/3]O-2. At an elevated temperature, amount of CO2 increases due to electrolyte decomposition. Li2MnO3 shows poor cycle performance, which is attributed to phase transformation and low charge-discharge efficiency during cycling. Low first-cycle efficiency, gas generation, and poor cycle performance limit the usage of Li2MnO3 in practical batteries.