The synthesis method of lithiated d-metal oxides using molten formate mixtures as precursors has been developed and the isothermal (800 °C) cross section of pseudo ternary Li–Mn–Co oxide system under ambient oxygen pressure has been investigated by XRD, 7 Li NMR, and galvanostatic electrochemical methods. Special attention has been paid to the compositions inside the quadrangle restricted by solid solutions LiCoO 2 –LiCo 0.85 Mn 0.15 O 2 with the layered structure of α-NaFeO 2 and solid solutions LiMn 2 O 4 –LiMnCoO 4 with the structure of spinel. It was found that, depending on the composition, three types of equilibrium phases could be formed: spinels LiLi,Mn,Co 2 O 4 with a part of Li atoms in octahedral sites, cation-deficit layered compounds Li 1 −  δ Co,MnO 2 , and Li 2 MnO 3 . Areas of (co)existence of these phases were plotted on the composition plane of the pseudo-ternary Li–Mn–Co system. Electrochemical properties of the compositions inside the quadrangle LiCoO 2 –LiCo 0.85 Mn 0.15 O 2 –LiMn 2 O 4 –LiMnCoO 4 are determined by the content and average oxidation number of Mn atoms, which is higher than in the normal spinels LiMn,Co 2 O 4 . Thus, the specific capacities of the polyphase compositions are lower in comparison with the binary solid solutions LiMn,Co 2 O 4 or pure LiCoO 2 .