Single crystals of silicon were irradiated with neutron doses ranging from 2.5 × 1018 n cm-2 up to 2.5 × 1019 n cm-2 (E>0.1 MeV). Diffuse X-ray scattering from the irradiated and isochronal annealed samples was measured near the (440) reflection. There are two distinct annealing stages, one at 200°C and another at 600°C. From the variation of the size and the shape of the interstitial and vacancy clusters we identify the mobile defects. We conclude that at 200°C the divacancy is migrating while at 600°C interstitials are mobile. Thereby, single interstitials being evaporated is more likely than complete interstitial clusters becoming mobile.