Indirect methods of investigation of composition and defect structures of lithium niobate (LiNbO₃) single crystals with different compositions are discussed. The analysis of two methods for the determination of the Li/Nb ratio in the samples is carried out, viz., the fundamental UV absorption edge and IR vibrational spectra of the OH-group defects. Intrinsic defect concentrations in lithium niobate crystals (lithium vacancies, V _Li ^-, and defects, Nb_Li ⁴⁺) as a function of the Li/Nb ratio in the samples are given. The results obtained can serve as an effective way of express non-destructive composition analysis in a mass production of parallel-plane plates.