Nanocrystalline Fe(1-x)Cox (x = 0.12, 0.5, 0.6) mixtures have been prepared by mechanical alloying using a planetary ball mill under several milling conditions. The alloying process between the elemental powders and the microstructure changes of the prepared samples have been investigated using X-ray diffraction (XRD) and Fe-57 Mossbauer spectrometry. The XRD patterns refinement on the mechanical alloying products according to the Rietveld method (Maud software) reveals the allotropic transformation of Co from fcc to hcp form and the progressive dissolution of Co into the alpha-Fe lattice as a function of both Co concentration and milling conditions. The grain size values of the obtained powders reaches a nanometer scale of about 10 nm. Mossbauer results show that the beginning of the solid state reaction between Fe and Co elemental powders depends on the milling conditions. After 24 h of milling, the average hyperfine magnetic fields values of Fe88Co12, Fe50Co50 and Fe40Co60 mixtures are consistent with a disordered and an ordered FeCo solid solution, respectively. (C) 2004 Kluwer Academic Publishers.