Intermolecular/interionic vibrational spectra of mixtures composed of 1-methyl-3-n-octylimidazolium tetrafluoroborate ionic liquid and benzene at mole fractions of 0 (i.e., neat ionic liquid), 0.2, 0.4, 0.6, 0.8, and 1.0 (i.e., neat benzene) have been investigated using femtosecond Raman-induced Kerr effect spectroscopy. The line shape of the low-frequency Kerr spectra obtained from Fourier transform analyses of the Kerr transients is strongly dependent on the composition of the mixture. By comparing the experimental spectra to calculated spectra based on experimental spectra of the neat liquids, it is evident that the spectrum is not achieved by simply combining those of the neat liquids and taking the mole fraction into consideration. Close examination of the spectral comparison results in a microscopic picture involving specific stacking of imidazolium and benzene rings that is not sufficiently stable to affect the ring librations. The quantum chemistry calculation results also support this proposal. No clear correlation between the first moment of the spectrum and the bulk parameter (i.e., the square root of the surface tension divided by the liquid density), which occurs for neat liquids, is evident.