X-ray and neutron diffraction studies were carried out on highly ordered (2−3% Li/Ni exchange) layered Li1-xCo1/3Ni1/3Mn1/3O2 [(1 − x) = 1.0; 0.8, 0.6, 0.45, 0.30, 0.04] prepared by chemical delithiation and relithiation. The studies reveal that the initial R3m phase (O3) is maintained up to extraction of 0.70−0.75 Li and exhibits only ∼1% change in volume over the range of composition. Additional extraction of Li results in the appearance of the O1 phase (trigonal; P3m1), which displays a strong contraction in the c axis by 5.3% and a volume decrease of 7.2%. Oxygen vacancies were not evident in the completely delithiated material. Re-intercalation of lithium in this phase was relatively irreversible, however, resulting in poorly defined mixtures of the O3 and O1 structures that exhibited a large fraction of stacking faults. In contrast, materials that were not delithiated beyond x = 0.30 resulted in fully reversible reformation of the original crystalline O3 phase on re-intercalation. Electrochemical ...