The optical stability of the R-lines and broad band associated with the Cr3+ ions has been studied in codoped congruent and near stoichiometric LiNbO3:Cr:Mg crystals. It has been found that the E-2 -> (4)A(2) transition of the Cr3+ ions located in Nb5+ sites, [Cr](Nb), can be bleached under optical excitation of the high energy broad absorption band luminescence whereas the emission from the same transition but ascribed to Cr3+ ions located in Li+ sites, [Cr](Li), is optically stable. At variance, the broad emission associated with the T-4(2) -> (4)A(2) vibronic transition ascribed to these centres is optically stable. The recovery temperature for the optical bleaching has been found to be 150 K, and has been correlated to Nb4+ electron traps located in Nb5+ antisite position. The different optical stability found for the R-lines and broad band emission associated with the [Cr](Nb) sites confirm the existence of two [Cr]Nb centres with low and high crystal field, respectively. (c) 2006 Elsevier B.V. All rights reserved.