A study has been made of the absorption bands of silicon, irradiated with high doses of fast neutrons (E≳1 MeV), by infrared spectroscopy in the region between 200 and 4000 cm−1(50–2.5 &mgr;). The relationship between several effects [band tailing (0.2–0.5. eV), divacancy‐associated band (3.45 &mgr;m), higher‐order bands (700–1400 cm−1), and single‐phonon bands (observed at 488, 410, and 332 cm−1)] and different irradiation doses up to 1019fast neutrons/cm2has been investigated. From the results of the absorption due to band tailing it can be assumed that the crystal is not amorphous. However, the observed vanishing of the higher‐order bands at the highest doses indicates an important change in the damaged lattice. The annealing behavior up to 600 °C, at the measuring temperatures of 78 and 300 °K, showed a slower annealing out of the single‐phonon bands for the high dose. From the stressing experiments, performed for single‐phonon bands, a constant dichroic ratio of 1.12 was measured. From this an estimation of the radiation‐induced strain within the lattice was made and found to be ?2×10−2. A new electronic band was found after a long‐term annealing at the wave number 312 cm−1.