Based on descriptions of the previously assessed sub-systems, we present a thermodynamic description of the La–Mn–Y–Zr–O system. Ideal extrapolations from ternaries to quaternaries and to La–Mn–Y–Zr–O are proved to be quite successful. The calculated isothermal sections of pseudo-ternaries at different temperatures in air are in good agreement with available experimental data. The thermodynamics at the LaMnO 3–yttria-stabilized zirconia (YSZ) interface, which is of particular importance for solid oxide fuel cell (SOFC) applications, is discussed in detail. Based on phase equilibrium calculations, we conclude that La 2Zr 2O 7 forms at the LaMnO 3–Y SZ interface due to its high chemical stability. Thermodynamically, the La 2Zr 2O 7 formation cannot be prevented by increasing the La deficiency in La 1± x MnO 3±δ. Increasing the yttria content in YSZ can suppress and in the end prevent the La 2Zr 2O 7 formation. Apart from the temperature and the oxygen partial pressure, other factors, like the phase assemblage at the interface, the La deficiency in La 1± x MnO 3±δ, the yttria content in YSZ and so on, also influence the amount of LaO 1.5 or MnO x (from LaMnO 3) dissolved in YSZ.