Fe100−xNix powders (22.5≤x≤40 at.%) mechanically alloyed (MA) for 10 h were characterized by x-ray diffraction (XRD) and transmission 57Fe Mössbauer spectrometry (TMS). In all of this composition range, the nanostructured alloys consist of two crystalline phases, body-centred cubic (BCC) and face-centred cubic (FCC). The Mössbauer spectra were fitted by means of a new fitting model involving two hyperfine magnetic field distributions (HMFDs), and a narrow singlet. One HMFD corresponds to the ferromagnetic BCC grains (tetrataenite), and the other to the ferromagnetic FCC grains (taenite), and the narrow singlet to paramagnetic FCC grains (antitaenite or superparamagnetic FCC grains). The Ni content dependence of the hyperfine field at 57Fe nuclei of the FCC phase gives evidence for some jump at about 32.5 at.% Ni, attributed to Invar anomaly.