Lithium-manganese-cobalt oxides, LiMnyCo1-yO2, where Li/(Mn + Co) approximate to 1 and 0 < y < 1, have been obtained by a solid state reaction between lithium hydroxide and manganese-cobalt spinels in air (0 < y < 0.2) or under nitrogen (0.2 < y < 1). The crystal structure of the ternary oxides depends on the Mn/(Mn+Co) ratio : up to y < 0.2, oxides with a trigonal structure (s.g. R $($) over bar$$ 3m) are formed; at 0.2 < y < 0.7, oxides with a rock-salt type structure (s.g. Fm $($) over bar$$ 3m) appear; finally, at y > 0.7, the oxides obtained have a tetragonal spinel structure (I4(1)/ amd). Acid delithiation of the lithium-manganese-cob alt oxides proceeds differently depending on their composition and crystal structure. With cobalt-rich oxides, the acid extraction of lithium proceeds within the initial trigonal structure, while with rock-salt and tetragonal oxides acid delithiation yields Li-x(MnyCo1-y)O-2 oxides with a cubic spinel structure (s.g. Fd3m). Among LiMnyCo(1-y)O(2) oxides, lithium is removed more easily and to a higher extent from the tetragonal oxides. Due to the high oxidation degree of the manganese and cobalt ions in delithiated samples, the electrochemical cell consisting of Li/Li-x(MnyCo1-y)O-2 displays a high discharge voltage (4.1-4.2 V), which allows its direct discharge. The highest reversibility and capacity per formula unit (about 0.75 Li) are observed on manganese-rich spinel oxides with y > 0.7.