A complete thermodynamic description of the Na2O–FeO–Fe2O3–Al2O3 system was conducted based on the critical evaluation and optimization of available thermodynamic properties and phase diagram data. The thermodynamic behavior of the liquid solution was described using the Modified Quasichemical Model, and the solid solutions Na(Al,Fe)O2 meta-oxides and Na2(Al,Fe)12O19 Na-β″-alumina were modeled using the Compound Energy Formalism considering their sublattice structures. Most experimental phase diagram data at reducing and oxidizing conditions were reproduced within 3 mol% of compositional error ranges. Many unexplored phase diagrams of this system under different oxygen partial pressures were predicted within the estimated accuracy of ±5 mol% in composition and ±50 K in temperature from the present thermodynamic models with optimized parameters.