The concentration of hydroxyl (–OH) groups in epitaxial barium titanate (BaTiO3) films (thickness ∼ 200 nm), deposited on single-crystal strontium titanate (SrTiO3) at 150 °C by a hydrothermal technique, was investigated using x-ray photoelectron spectroscopy and Fourier transform infrared (FTIR) spectroscopy. After hydrothermal treatment, a broad FTIR resonance for the hydroxyl groups indicated a significant concentration of surface –OH groups in the films. The as-deposited films were subsequently treated hydrothermally with D2O, and the kinetics of the exchange reaction between –OH incorporated into the film and –OD from the D2O were studied using FTIR. For reactions carried out intermittently, the kinetics of the exchange reaction between –OH by –OD depended not only on the total reaction time, but also on the duration of each treatment. The broad FTIR hydroxyl resonance in the as-deposited hydrothermal film was significantly reduced only after heating for 1 h at 600–800 °C.