The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3+ sites; Cr3+ ion substituting Li+ and Nb5+ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3+ ions located in Nb5+ site ([Cr](Nb) centres). The observation of the zero-phonon line at 13 540 cm(-1) only in pure LiNbO3:Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals. (C) 2003 Elsevier Science B.V. All rights reserved.