Vibrational spectroscopic studies have been carried out on solutions of LiBF4 in acetonitrile as a function of concentrations of lithium salt. Great changes in the vibrational characteristics of CdropN and C-C stretches were observed as a result of solvation. The solvation numbers of Li+ determined from spectroscopic data are found to decrease from 3.2 to 1.4 with increasing, concentration of the lithium salt. It is expected that the solvation number is 4 in the infinite diluted solution. Solvation structures of the lithium ion were suggested by density functional theory and the results were compared with the spectroscopic data. Changes of the v(1) mode of BF4- with concentration of the salt were also analyzed, and the bands at 763, 771, and 780 cm(-1) indicate the coexisting of free ion, contact ion pairs, and dimers. The structures of ion pairs in gas phase and in acetonitrile solutions were suggested on the basis of DFT calculations.