Li ion kinetics in the O2-phase layered manganese oxides, Li2/3(Co0.15Mn0.85)O2 (O2(Li)) and Li(2/3)+x (Co0.15Mn0.85)O2 (x=1/3 (O2(Li+x))), has been studied by the electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) at room temperature and the results were correlated with the observed cathodic behaviour. Both compounds show a reversible capacity of ∼180 mA h/g at low current density (∼5 mA/g). EIS studies as a function of cycle number show an increased contribution of resistance associated with surface film formation and bulk contribution which is in agreement with the increased capacity fading observed in O2(Li+x) after 10–15 cycles. The Li ion diffusion coefficient (D Li) vs voltage plots show minima during the first charge cycle coinciding with the irreversible plateau of the voltage vs capacity profiles reflecting the irreversible phase change in both the compounds. The values of D Li (GITT method) observed for the second and subsequent cycles (≤6) in the full voltage range (3.0–4.4 V) are 2×10−11–10×10−11 cm2/s for O2(Li+x) and 0.5×10−10–3.0×10−10 cm2/s for O2(Li). Variation of D Li as a function of cycle number (up to 35) indicates that, in addition to the interface kinetics, changes in the D Li values with cycling also contribute to the capacity fading of the compounds, especially in O2(Li+x).