We report here the low-frequency spectra, resulting from the intermolecular/interionic vibrational dynamics, of aqueous mixtures of an ionic liquid, 1-methyl-3-n-octylimidazolium tetrafluoroborate, with the H2O mole fractions of 0.2, 0.4, and 0.6 and the neat ionic liquid and H2O within the frequency range of 0.1-700 cm(-1) by means of femtosecond Raman-induced Kerr effect spectroscopy. Addition of H2O induces tiny effects on the line shape of the low-frequency Kerr spectrum of the ionic liquid: ca. a 2 cm(-1) red shift in the first moment of the low-frequency spectrum has been observed for a transition from the neat ionic liquid to the binary mixture containing 0.6 mol fraction of H2O. Surface tension and liquid density of the mixture also accompany minimal changes upon addition of H2O. These results suggest that H2O molecules localize at the interface between the ionic and nonpolar regions, and the interionic interaction in the ionic. region is weakly perturbed by the existence of H2O. On the other hand, successive addition of H2O in the mixture slows down the picosecond overdamped relaxation process measured in the 3-300 ps range even though the shear viscosity of the mixture decreases substantially.