The structure and the electrochemical properties of P2-Na-2/3[CoxNi1/3-x/2Mn2/3-x/2]O-2 and Li-2/3[CoxNi1/3-x/2Mn2/3-x/2]O-2 (0 less than or equal to x less than or equal to 2/3) are investigated by X-ray and neutron diffraction as well as by electrochemical techniques. Li-2/3[CoxNi1/3-x/2Mn2/3-x/2]O-2 (0 less than or equal to x less than or equal to 2/3) can be considered a solid solution between Li-2/3(1+)=[Ni1/32+Mn2/34+]O-2(2-) and Li-2/3(1+)[Co2/33+Mn1/34+]O-2(2-). Samples with x near 0 and with x near 2/3 adopt the T2 structure, while those with intermediate x show a stacking faulted O2 structure. This paper demonstrates that T2-Li-2/3[Co2/3Mn1/3]O-2 is quite different from T2-Li-2/3[Ni1/3Mn2/3]O-2 in its properties. First, neutron diffraction studies indicate that there is no superlattice ordering of Co and Mn in the transition metal layers of P2-Na-2/3[Co2/3Mn1/3]O-2 or T2-Li-2/3[Co2/3Mn1/3]O-2, as there is in P2-Na-2/3[Ni1/3Mn2/3]O-2 and T2-Li-2/3[Ni1/3Mn2/3]O-2. Second, the electrochemical results show that all the lithium can be extracted from T2-Li-2/3[Co2/3Mn1/3]O-2, unlike T2-Li-2/3[Ni1/3Mn2/3]O-2, from which only one half the available lithium can be extracted.