Optical absorption of electronic defects induced by chemical reduction, and chemical diffusion in a vapor transport equilibrated lithium niobate single crystal with 49.5 mol% Li2O were studied using UV-vis-NIR optical spectroscopy under in situ conditions. In reducing atmospheres at 1000 degrees C, optical absorption spectra are dominated by a band at about 0.93 eV which is attributed to absorption of free electron-polarons, i.e., to electrons localized on niobium ions on regular lattice sites. The electron-polaron band intensity was found to follow a power law of the form (pO(2))(m) with m approximate to -0.21. Chemical reduction reactions forming free electron-polarons were found to explain the observed defect absorption and its dependence on oxygen partial pressure at high temperatures. The chemical diffusion coefficient in the vapor transport equilibrated lithium niobate has been determined to be 2.75 x 10(-11) m(2)/s at 1000 degrees C from the kinetics of reduction and oxidation processes in the range of -10 < log (p(O2) /atm) < -14. (C) 2013 Elsevier B.V. All rights reserved.