Thin Cr films were nitrided during rapid thermal annealing for 3 min in NH 3 between 500 and 850°C. Nitridation begins at 500°C and occurs in a layer-by-layer fashion, first forming Cr 2 N then CrN. At 550°C, Cr 2 N begins to nitride to CrN despite the existence of unreacted Cr, such that a CrN/Cr 2 N/Cr structure exists. At 650°C, no unreacted Cr remains and the layer consists of CrN on Cr 2 N; this configuration remains at 700°C although CrN grew at the expense of Cr 2 N. At 750°C, the reaction is complete, forming CrN. Oxygen accumulates in front of the growing nitrides until the temperature is high enough to allow redistribution. This occurs at lower temperatures in Cr 2 N than in CrN, which may be correlated to the enhanced oxidation resistance of CrN. Phase identification was performed by X-ray diffraction, composition determination by Rutherford backscattering spectrometry, and elemental profiling by secondary ion mass spectrometry. Ion beam resonances of the forms (α,α) 1 6 O(α,α) 1 6 O and (α,α) 1 4 N(α,α) 1 4 N provided additional elemental profiling. Vickers microhardness was measured and was observed to increase sharply once a surface nitride had formed, then remained nearly constant upon further annealing.