The structural state at a Fe/GaAs interface has been investigated after ion-beam mixing (IBM) using X-ray diffraction, transmission electron microscopy and conversion electron Mössbauer spectroscopy. 50 and 100 nm thick Fe films were deposited on (100) GaAs substrates and the IBM was carried out at 293 K with 320 keV Kr 2+ ions to doses from 1 to 8 × 10 16 ions/cm 2 . The as-deposited films consisted of α-Fe and after mixing an amorphous FeGaAs phase appeared in the subsurface zone. Different Fe coordination and magnetic states have been identified in the mixed layer and attributed to ferromagnetic Fe, paramagnetic Fe and paramagnetic Fe-oxide respectively, the former phase being the most abundant. The irradiation-dose growth led to a gradual decrease of α-Fe content and a monotone increase of amorphous phase content. For the 50 nm thick Fe films, complete mixing was obtained at a critical fluence of 5 × 10 16 ions/cm 2 . A study of mixing efficiency performed on 100 nm thick Fe films showed that the efficiency was somewhat less than 1, thus suggesting the onset of demixing processes.