Layered lithium intercalation materials Li[NixLi 1/3-2x/3Mn2/3-x/3]O2 with O3 structure are based on the layered compound Li2MnO3 (Li[Li1/3Mn2/3]O2) in which 1 Mn4+ and 2Li+ ions are replaced by 3Ni2+. On initial deintercalation of lithium, Ni2+ is oxidized to Ni4+ but further lithium removal is possible. Two mechanisms have been considered to explain the ability to overcharge, simultaneous oxygen loss or H+ exchange the latter are generated from decomposition of the electrolyte. We show that the dominant mechanism of Li overcapacity during electrochemical charging is oxygen loss, in agreement with Dahn's results. Some H+ exchange does occur, however, and this is greater at 55°C than at 30°C, accounting for about 25% of the excess lithium removed at the higher temperature. In contrast to electrochemical charging, chemical removal of lithium using NO2BF4 is accompanied by more H+ exchange than oxygen loss.