Lithium-manganese-cobalt oxide, Li(Co1-xLix/3Mn2x/3)O-2 (0 less than or equal to x less than or equal to 1) was prepared as a solid solution between the two kinds of layer structured end members, LiCoO2 and Li2MnO3. Simple calcination of the stoichiometric mixture of the oxide constituents gave a mixture of two end members. Excess lithium carbonate or Lithium hydroxide should be added to the mixture on calcination at 900-1000 degrees C to obtain the solid solutions. The excess lithium compounds were removed by washing with water. The results of the X-ray diffraction studies, chemical analysis and XANES measurements suggested that the Co in LiCoO2 was substituted for (Li1/3Mn2/3). The solid solutions were electrochemically active as cathode materials for a lithium battery, though the discharge capacity decreased with the increase of x in Li(Co1-xLix/3Mn2x/3)O-2.