Abstract. Solution of a multifaceted problem of effective materials production for to-date technologies, along with the development of methods for the synthesis of required chemical and phase composition substances, must ensure the reproducibility of specified functional properties (electrochemical, magnetic etc.) thereof. The latter is of particular importance in the case of a real multicomponent material with different volatility of the components it consists of. Slight temperature or pressure fluctuations of the volatile component during synthesis procedure can result in considerable effect on the elemental and phase composition of the matter, and hence affect its functional properties. Deviations from the initial composition can also occur under the operating conditions of this material. Thus, the solution of this complex problem should also include the study of phase equilibria (both stable and metastable) in a given multicomponent system in a wide range of temperature, concentrations of components, and partial pressure of the volatile component P(X). Current report summarizes the results on authors’ experimental studies of phase equilibria in the quaternary system Li-Mn-Eu-O, as well as related ternary oxide systems with Mn and Eu in polyvalent states, that include crystalline phases and solid solutions of particular technological interest for the development of Li-ion batteries cathodes, catalysts, optical and magnetic materials