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We have measured the refractive indices of Mg : LiNbO 3 crystals before and after chemical reduction at different annealing temperatures by the spontaneous parametric down-conversion technique and nearforward Raman scattering by polaritons in the range 2-11 µm. The polaron-induced contribution to the dispersion of the real part of the dielectric function was revealed near the phonon absorption edge. It was found that this contribution increases considerably with increase in the polaron concentration.