The reaction mechanism and kinetics of the thermal decomposition of uranium dinitride/uranium sesquinitride to uranium mononitride under inert atmosphere at elevated temperature were studied. An increase in the lattice parameter of the UN2/alpha-U2N3 phase was observed as the reaction temperature increased, corresponding to a continuous removal of nitrogen. Electron density calculations for these two compounds using XRD powder patterns of the samples utilizing charge-flipping technique were performed for the first time to visualize the decrease in nitrogen level as a function of temperature. Complete decomposition of UN2 into alpha-U2N3 at 675 degrees C and the UN formation after a partial decomposition of alpha-U2N3 at 975 degrees C were also identified in this study. The activation energy for the decomposition of the UN2/alpha-U2N3 phase into UN, 423.8 +/- 0.3 kJ/mol (101.3 kcal/mol), was determined under an inert argon atmosphere and is reported here experimentally for the first time.