The gas nitriding of nanocrystalline iron was studied at 400 degrees C under atmospheres with different nitriding potential. Three iron samples of various mean size of crystallites were used. They were nitrided in gas mixtures with NH3/H-2 ratio increasing stepwise up to the pure ammonia. Then, the obtained iron nitrides were reduced in gas mixtures with NH3/H-2 ratio decreasing stepwise down to the pure hydrogen. The variation of mass gain was observed gravimetrically and the phase composition was examined by XRD analysis. The higher the mean size of crystallites the lower nitriding potential was required to initiate the phase transition alpha-Fe -> gamma'-Fe4N observed during the nitriding process. The observations are explained with the application of the Gibbs-Thomson effect to the nanocrystalline Fe-N system. (C) 2013 Elsevier B.V. All rights reserved.