We have investigated the effect of deuterated water on the conformational equilibrium between the gauche and trans conformers of the [bmim] cation in mixtures of water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), an ionic liquid, at room temperature. A comparison of the results obtained from solutions made with H2O and with D2O highlights an anomalous conformational change in the D2O solution showing an extended N-shaped behavior. The gauche conformer of the [bmim] cation in D2O increased up to x = similar to 50 (D2O mol %); however, it decreased up to higher water concentrations of x = similar to 85 before again increasing drastically toward x = similar to 100. We provide spectroscopic evidence that the anomalous conformational dynamics of the [bmim] cation in D2O is directly related to the H/D exchange reaction of the C-H group at position 2 of the imidazolium ring.