The infrared spectroscopic profiles of H2O/D2O mixtures have been analyzed on the application of pressure up to 4 GPa. As the mixtures transform to high-pressure ices, blue-shifts of the bending peak frequencies were observed. interestingly enough, the bending frequency shift ratio is about 0.8:1.9:3.0 for D2O/HOD/H2O, respectively. As the refractive index and elastic constant show no distinction between H2O and D2O over our measured pressure range, this study demonstrates that the bending vibrations of D2O, HOD, and H2O provide the key to the understanding of isotopic intermolecular interactions of aqueous equilibrium mixtures.