Lithium-rich transition-metal oxides lose oxygen during the first removal of lithium. In this study, the kinetics of the oxygen-loss process of the lithium-rich oxide, Li[Li1/9Ni1/3Mn5/9]O-2, of varying particle sizes was studied. The electrochemical data were analyzed, and oxygen diffusion constants of 3x10(-13) and 2x10(-12) cm(2) s(-1) were determined for the oxygen-loss process at 30 and 55 degrees C, respectively. This is about 1000 times smaller than typical lithium diffusion constants in layered oxides. The Li[Li1/9Ni1/3Mn5/9]O-2 material with a 3 mu m particle size was cycled without capacity loss at a specific capacity of 240 mAh g(-1) at 55 degrees C.