The optical constants of some imidazolium-based ionic liquids (ILs) are determined in the mid- and far-infrared regions by combining polarized attenuated total reflection (ATR) and transmittance spectra. The internal vibrations of the cations and anions and the interionic vibrations can thus be quantitatively evaluated. A comparison of the far-IR spectral response of several imidazolium derivatives associated with the (CF3SO2)(2)N- anion shows that methylation of the more acidic C(2)H imidazolium group does not change the far-IR intensity and hence that the CH center dot center dot center dot anion hydrogen bonds play a negligible role compared with electrostatic interactions. The calculated spectra of ion-pair dimers reproduce the far-IR density of states better than those of simple ion pairs.