The surface layer of LiNbO3 congruent crystals was studied at room temperature and the 300-1000 degrees C temperature range. The XPS test discerned two temperature ranges with different electronic structures and chemical compositions of the crystal sample surface layer. We observed a deficiency of Li ions in surface of as-received epi-polished samples, at RT, and in range from 300 to 500 degrees C. The concentration of Li ions in epi-polished surface did not exceed 4%, which was much lower than the Li concentration determined for cleaved crystal, at RT. We explained this Li shortage with the use of leaching procedure, which was used in the crystal sample preparation stage. The XPS spectra, which have been obtained in the temperature of 300-500 degrees C, have shown three oxidation states of niobium Nb 3d(0), 3d(1), 3d(3), broadening of O 1s, and occurrence of C is core lines. In higher temperature range (700-1000 degrees C), we detected an increased concentration of lithium and changes in the oxidation state of niobium that indicated congruent composition in the surface of lithium niobate crystal. The conditions, which corresponded to migration of Li ions, have been checked with the use of thermogravimetric analysis, effusion test, and TOF-SIMS spectroscopy. Thermogravimetry has shown mass losses of bulk crystal and powdered samples. The powdered samples exhibited saturation of mass loss at reducing conditions. The effusion experiments confirmed that O and Li2O were the most volatile species in LiNbO3. Mass spectroscopy (effusion and TOF-SIMS) shown out-diffusion of lithium ions from the deeper part of the crystal to the surface layer. TOF-SIMS maps exhibited a non-homogeneous composition and granular structure of distribution of the Li and Nb elements on the surface of the reduced sample. (C) 2016 Elsevier B.V. All rights reserved.