Nanocrystalline iron (17 nm) was obtained by fusion of magnetite with Al, Ca, and K oxides, followed by the reduction of the alloy, As well as iron, the obtained specimen contained 2.92 wt. % Al2O3, 2.97 wt.% CaO, 0.65 wt.% K2O, 0.31 wt. % SiO2 and 1 wt.% of oxides of other metals (Mg, Ni, Cr, Ti, V). The sp. surface of the reduced and passivated samples was 11 m(2)/g. The iron specimen was carburised with 1.2 dm(3)/g/min of CH4 (or CH4/H-2 or CH4/N-2) over 500-600degreesC to yield Fe3C or nitrided with 1.0 dm(3)/g/min NH3 (or NH3/H-2) over 250-500degreesC to yield FexN (x = 4, 3, 2). The carburization and nitriding processes were followed using thermogravimetric studies. Neither external nor internal diffusion processes affected the formation rates. Over the rectilinear segment of the TG line, the Fe3C formation was first order with respect to CH4 partial pressure. The apparent activation energies of formation of Fe3C and Fe4N were 158 and 40 kJ/mol, resp., regardless of the Fe surface chemical composition. As the degree of conversion increased, the average Fe crystallite size rose, contrary to the shrinking and crackling core models. A new model of the nucleation was proposed, involving the increasing order of the size of the converted iron crystallites, from the smallest to the greatest ones. Based on this nucleation model, the size distribution of iron nanocrystallites was determined.