We have observed a strong correlation between the internal fields and the infrared absorption spectra of OH−ions in congruent LiTaO3crystals. After reversal of the direction of polarization (Ps) of the crystal by an electric field at room temperature, (a) the internal field is antiparallel toPs, (b) the relative intensities of the infrared absorption bands of OH−ions (at ∼3462, ∼3476, and ∼3490 cm−1) change, and (c) the 180° domain walls are visible under a light microscope. Annealing the domain‐reversed crystal at 200 °C or above and cooling back to 25 °C has the following effects: (a) The internal field realigns parallel toPs. (b) The shape of the OH−spectrum returns to the shape observed before domain reversal. The time–temperature dependence of the shape of the infrared absorption spectrum of OH−ions also shows the same characteristics as that of the realignment process for the internal field. (c) The optical domain wall contrast becomes extremely weak. Infrared absorption studies suggest that domain inversion as well as the heating of LiTaO3crystal induce structural changes that are similar in nature. It is proposed that the origin of internal fields lies in OH−ions and/or nonstoichiometric point defects in the LiTaO3crystals. We also explain the optical 180° domain wall contrast based on internal fields. ©1996 American Institute of Physics.