The stoichiometric iron nitrides γ′‐Fe4N, ε‐Fe3N and ζ‐Fe2N were characterized by Mössbauer spectroscopy. The thermal decomposition of ε‐Fe3N was studied in‐situ by means of a specially developed Mössbauer furnace. We found ε‐Fe3N to γ′‐Fe4N and ε‐Fe3Nx (x ≥ 1.3) as decomposition products and determined the border of γ′/ε transformation at T ≅ 930 K. Mössbauer spectroscopy was applied to study in‐situ the thermal decomposition of the nitridometalate Li3[FeIIIN2] and the formation of Li2[(Li1‐xFeIx)N], the compound with the largest local magnetic field ever observed in an iron containing material. The kinetics of formation and the stability of Li2[(Li1‐xFeIx)N] was of particular interest in the present study.