Two optical methods for determination of the composition in lithium tantalate were discussed. Quantitative relationships between the line width of Raman peaks (142 cm(-1) for E phonon and 861 cm(-1) for A(t) phonon) and the crystal composition were firstly presented. The comparison of the Raman data obtained for the congruent and the near-stoichiometric crystals reveals some differences in the shape and the number of Raman peaks, which lead to a new assignment of the long-wavelength optical phonons. The OH- absorption bands of lithium tantalate crystals were measured at room temperature, and the line width of one decomposed peak was found to change considerably with the crystal composition, irrespective of the hydrogen concentration. The quantitative relationship between the line width of this peak and the crystal composition is also presented. (c) 2005 Elsevier B.V. All rights reserved.