We determined the complex dielectric constants of the liquid water isotopes, H2O, D2O, and H-2 O-18 at 296 K from 0.2 to 7 THz by terahertz time-domain attenuated total reflection spectroscopy. We extracted the dispersion of the complex susceptibility of the intermolecular stretching vibration mode around 5 THz. The isotope effect on the vibration mode is clarified in its amplitude, frequency, and damping constant. Anticorrelation of the oscillator strength of the intermolecular stretching vibration mode between infrared activity and Raman activity indicates that H2O has the strongest structural disorder among water isotopes. (c) 2009 Elsevier B.V. All rights reserved.