Thermodynamic modeling of phase equilibria in a liquid metal of Fe–La–Ce–O system at 1600 °С, using the technique of constructing the solubility surfaces for the components of a metal, was carried out. The calculation technique allowed assessing the depth of liquid iron de-oxidation at a complex use of lanthanum and cerium as deoxidizing agents. Also, diagrams of de-oxidants’ consumption for one ton of liquid oxygen-containing iron were calculated in the course of the work. Carrying out a calculation of the solubility surfaces for the components of a metal required simulation of phase diagrams of the following oxide systems: FeO–La2O3–Ce2O3, FeO–CeO2–La2O3, CeO2–La2O3–Ce2O3. The obtained results might be of interest for optimization of the use of rare-earth metals in steelmaking technology.