Process for nanocryst. nitriding of alpha-iron to gamma'-iron nitride in NH3 atm., at 350 degrees C and under atm. pressure, was carried out using a chem. potential programmed reaction method, which was implemented in 2 measuring systems: gravimetric and X-ray diffraction in situ. According numerical modeling, 2 non-identical crystallite size distributions (CSD) were obtained. The differences between CSD obtained with 2 other techniques were explained as an effect of disparities in chem. and phase compn. of the sample in the nitriding process. The sample tested had a bimodal size distribution of crystallites with ay. crystallite size of 46 and 68 nm.