The layered rock salt material Li-2/3[Mn2/3Ni1/3]O2-delta with the O3 structure loses up to 16 2/3 % of its oxygen reversibly at high temperature. It transforms initially to a second layered structure, O3('), that may be associated with cooperative Jahn-Teller distortion of Mn3+ ions and considerable interlayer cation mixing. By continuous disordering and further oxygen loss, it finally transforms into a simple cubic rock salt structure above 1000 degrees C, as shown by thermogravimetry and high-temperature X-ray powder diffraction. X-ray absorption near-edge structure results confirm that oxygen deficiency induces reduction of mainly Mn and to a small extent Ni, commencing with the fully oxidized stoichiometry Li-2/3[Mn2/33.5+Ni1/33+]O-2. This correlates with oxygen contents determined by thermogravimetry and Rietveld refinement of powder X-ray diffraction data, with oxidation states determined by chemical titration and with electrochemical behavior during cycling. The cycling results show that samples with the O2 structure have better performance and capacity than those with O3 and especially O3(') structures and that the O3, O3(') capacity decreases with increasing oxygen deficiency delta. (c) 2007 The Electrochemical Society.