Acoustic and optic phonons have been measured by Brillouin and Raman spectroscopies in LiNbO3 samples with different stoichiometries. Consequently, the elastic constants have been determined by Brillouin scattering as a function of the stoichiometry. The C-44 elastic constant presents, as a function of the stoichiometry, a clear minimum for the [Li]/[Nb]=1 in the melt sample. This behavior must be related to the existence and competition of two kind of defects, i.e., extended defects induced when growing samples not at the congruent composition, and point defects present in nonstoichiometric samples. The optic phonons seem to be dominated by the point defects. The linear increase of the phonon lifetimes, with respect to an ideal stoichiometric sample, can be due to the scattering by the static imperfections and to an increase of the anharmonic scattering. Several Raman peaks, which appear when the [Li]/[Nb] ratio decreases, are related to the presence of Nb in antisites. The width of some particular phonons can be used to estimate the [Li]/[Nb] ratio whenever a line shape study and a good correction, taking into account the instrumental broadening, is carried out. (C) 1996 American Institute of Physics.