Abstract <p>The thermodynamic modeling of uranium mononitride (UN) oxidation by gas mixtures (Ar + O₂) at different temperatures and oxygen contents in the mixture is performed. The oxidation is found to proceed via several consecutive stages, and each stage includes a number of parallel reactions. At the majority of stages, the equilibrium composition of oxidation products is complicated and includes nonstoichiometric compounds. The exception is the composition with the ratio O/U = 2 at which stoichiometric UO₂ is formed. The kinetics of UN oxidation by the (Ar + 20% O₂) gas mixture is studied. According to differential thermal analysis results, uranium mononitride is slowly oxidized in a temperature range of 300–400°C. As a temperature of 420°C is reached, the sample weight increases sharply accompanied by a significant heat evolution. The maximum reaction rate is achieved at 432°C. The maximum increase in the sample weight is 12.4%, which exceeds the theoretical value in the reaction UN → U₃O₈ (+11.26%) but is lower than that in the reaction UN → UO₃ (+13.49%). The sample weight begins to decrease with further heating above 500°C and decreases to the end of experiment by 0.5% of maximum values. Flue gases leaving a simultaneous thermal analyzer are examined using a quadrupole mass spectrometer. These gases, except for argon, oxygen, and nitrogen, contain impurities of nitrides (NO, NO₂, and, possibly, N₂O). The oxidation of UN by the (Ar + 20% O₂) gas mixture at 450°C is studied by thermogravimetry. The stages of the process are revealed. The maximum oxidation rate is 0.4%/min.</p>