An in-built internal field of 5 KV/mm was measured in Z-cut LiTaO 3 single crystals in the direction of the original polarization. This was reflected in the asymmetry (along the field axis) in the polarization hysteresis curve as a function of external electric field. Measurement of internal field as a function of crystal thickness revealed that the internal field is a volume effect. Using infrared absorption measurements, the spectrum of the hydroxyl ions (OH -) present in these crystals was measured at room temperature. The shape of the spectrum changes with polarization reversal in these crystals indicating a strong correlation with the direction of internal field with respect to the polarization direction. It suggests that the origin of internal field lies in point defect complexes involving OH - ions.