The complete solid solutions in the pseudo-binary LiNi1/3Co1/3Mn1/3O2-LiMg1/3Co1/3Mn1/3O2 system with alpha-NaFeO2 type layered rock-salt structure have been successfully synthesized. The replacement of Ni with Mg atoms has enhanced the diffraction intensity due to cation ordering in alpha-NaFeO2 type structure. Powder X-ray diffraction (XRD) patterns of LiMg1/3Co1/3Mn1/3O2 show a broad and diffuse peak with an intensity maximum at around d = 4.2 angstrom indicating an in-plane [root 3 x root 3] R30 degrees type ordering. Electron diffraction (ED) patterns also show clear and intense superlattice spots due to the in-plane [root 3 x root 3] R30 degrees type ordered layers. However, the [root 3 x root 3] R30 degrees type ordered layers are almost randomly stacked as evidenced by diffuse scattering in the powder XRD and ED patterns. The TEM lattice image clearly reveals the random stacking. XRD and ED patterns agree with the simulated ones using the DIFFaX program based on the above structural model. Solid solution LiNi1/3-xMgxCo1/3Mn1/3O2 (0.0 <= x <= 0.33) also show a similar two dimensional cation ordering. Electrochemical measurements of LiNi1/3-xMgxCo1/3Mn1/3O2 indicate that not only Ni but also Co can be active as redox species in this solution system. The redox potential of Co in LiMg1/3Co1/3Mn1/3O2 is about 4.1 V (C) 2007 Elsevier B.V All rights reserved.