AbstractIsothermal and dynamic TG were used to follow the oxidation of a sieved fraction, 177–250 μm, of niobium nitride powder in a flow of dry 0.1% O2 in Ar. The material underwent enormous expansion during the oxidation. The isothermal weight gain was measured in the temperature range from 817 to 897 K. It followed a pseudo‐first order reaction rate law best with an Arrhenius activation energy of 50 kcal mole−1. Dynamic studies at 1 K min−1 the pseudo‐first order rate law yielded similar values of 48 kcal mole−1 in 0.1% O2 in Ar and 55 kcal mole−1 in air. Because of its highly exothermic nature, the reaction will lead to a thermal runaway situation at heating rates approaching a temperature of 600 K at 20 K min−1, in air or at even lower rates of heating in pure O2, for material in this particle size range. Examination of the weight gain and X‐ray diffraction patterns of partially oxidized samples did not give any indication of intermediate species, such as oxynitrides or oxides of niobium, other than Nb2O5. The final product was a low temperature, pseudohexagonal polymorph of Nb2O5 but monoclinic γ'‐Nb2O5 appeared as an intermediate.