The OH - absorption spectra of low doped lithium niobate (LiNbO 3 ) crystals have been investigated. Though no apparent band shift is observed in these absorption spectra, their shapes are quite different. In order to analyze the information on the defect structure underlying these OH - absorption bands, the normalization and difference methods were employed. It was found that although the doping concentrations are under the thresholds the doping ions have apparent affect to the site occupation of OH - ions. The OH - vibrations related to Mg L i + (Mg 2 + occupying Li-site) and In L i 2 + are 3483 and 3484 cm - 1 in LiNbO 3 :Mg and LiNbO 3 :In crystals, respectively. The absorption peak of LiNbO 3 :Ti (2.5 mol%) crystal at 3487 cm - 1 is mainly related to Ti L i 3 + -OH - and the 3489 cm - 1 peak of LiNbO 3 :Mg (5.0 mol%), Ti (10.0 mol%) related to Mg L i + -OH - , Ti N b - -OH - and Ti L i 3 + -OH - . Doping with Na improves the peak intensity near 3466 cm - 1 and induces a new absorption peak at 3470 cm - 1 . The absorption bands of LiNbO 3 crystals codoped with trivalent ions are associated with the co-effect of the doped ions and have some different characteristics from mono-doped crystals.