Subsolidus equilibria in air in the NiO-CeO2 and La2O3-CeO2-Fe2O3 systems were studied. In the NiO-CeO2 and Fe2O3-CeO2 systems, no binary compound and no solid solubility were detected. La2O3 was soluble in CeO2, forming a cubic fluorite solid solution up to La0.5Ce0.5O1.75, whereas no solid solubility of CeO2 in La2O3 was detected. In the La2O3-CeO2-Fe2O3 system, no ternary compound was found. The tie line is between LaFeO3 and CeO2.