The vibrational spectra of acetonitrile solutions containing lithium iodide and tetrabutylammonium iodide have been studied using ATR-FTIR spectroscopy. The focus of interest was the effect of the iodide anion on the important vibrational bands of acetonitrile. The main effect of the Lil electrolyte is to shift the Cequivalent toN stretching frequency in the blue direction due to interaction of the Li+ cation with the electronegative Cequivalent toN group. However, a small red shift of the Cequivalent toN stretching mode due to the interaction of I- with the methyl group of acetonitrile is found in both electrolytes. On the other hand, the symmetic and asymmetric stretching modes of the methyl group are significantly red shifted in the presence of Lil. Ab initio quantum chemical calculations were carried out to determine the optimum location of the Lil and I- ions in ion-solvent complexes containing a varying number of solvent molecules. These calculations correctly predict the direction of the observed shifts for all the principal bands of acetonitrile except the methyl stretching modes.