The dependence of the optical absorption band edge, ordinary and extraordinary refractive index, powder second harmonic generation and the non-linear coefficient, d(33), of congruent LiNbO3 crystals doped with WO3 are reported. It has been found that the referred parameters show a clear singularity for a concentration in the range of 1.5-2%. In comparison with previous works on congruent LiNbO3 crystals doped with divalent and trivalent ions a charge compensation mechanism is proposed to explain the role of the cation. The experimental data of n. and n, are described by a generalised Sellmeier equation considering the different ions sites in the sample. (C) 2002 Elsevier Science B.V. All rights reserved.