The blue-shifted C-D stretching vibrations (nu(CD)) of deuteriated dimethyl sulfoxide (DMSO-d(6)) in mixtures of DMSO-d(6)/ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) were carefully investigated and compared with those in DMSO-d(6)/water. Results from FTIR studies indicate that the direct interaction of C-D center dot center dot center dot F between DMSO-d(6) and the anion [BF4-] plays a minor role in the blue-shift of nu(CD) in the mixtures of DMSO-d(6)/IL. They also show that the indirect influence of the hydrogen bond formed by the nearby functional group S=O with C-2-H of the cation significantly contributes to the blue-shift of nu(CD), as can be inferred from the significant differences of the blue-shift in [Bmim][BF4] and 1-butyl-2,3dimethylimidazolium tetrafluoroborate ([Bm(2)im][BF4]). In contrast, in the case of DMSO-d(6)/water mixtures, direct hydration of C-H center dot center dot center dot O between the methyl groups of DMSO-d(6) with the oxygen atom of water is mainly responsible for the blue shift of nu(CD). Furtherniore, theoretical calculations revealed that charge migrations as well as rehybridization have a dominant effect on the blue-shift in both mixtures.