The structure modification effect produced on a-Fe 30Ni 36Cr 12Mo 2B 15Si 5 under thermal treatment and neutron irradiation has been studied by the Mössbauer spectroscopy and by the low-field ac magnetic susceptibility measurements. Hyperfine field distributions, P( H), have been obtained from the Mössbauer spectra using modified Le Caer-Dubois histogram method. The average value and the width of the hyperfine field distribution as well as the Curie temperature value are found to increase after prolonged annealing while neutron irradiation induced atomic rearangement processes lead to considerable decrease of the above parameters value. The observed changes can be explained by, respectively, irradiation and annealing induced modifications of the local short range order around the magnetic atoms in the investigated samples.