The effect of X-irradiation on the absorption spectra of pure and impurity (Cu, Mn, Ni, Eu)-doped LiNbO 3 has been systematically investigated. Three main bands with peaks at 1.6, 2.5 and 3.2 eV have been observed in pure crystals or Mn-, Ni-and Eu-doped crystals after low-temperature (10K) irradiation. For the Cu-doped samples, a competition is apparent between the production of these bands and the conversion of Cu 2+(1.2 eV band) into Cu+(3.1 eV band). From this information and the thermal annealing behaviour of optical bands together with a comparison with the EPR data of Schirmer and Von der Linde (1978), the following assignment of optical bands has been made: the 1.6 eV band to polaron (Nb 4+) states; the 2.5 and 3.2 eV bands to holes trapped at Li vacancies or other lattice defects. The reduction treatments induce a broad absorption made up of two main bands at 2.6 and 3.2 eV. Optical bleaching experiments at 10K, turn these bands into the same 1.6 eV (polaron) band of irradiated crystals. For Mn-and Cu-doped samples those treatments induce, as a first stage, the reduction of Mn 3+ and Cu 2+ which is then followed by the formation of the same bands of pure crystals. These results have led us to associate both bands with F centres (2.6 eV) and F+ centres (3.2 eV).