We have demonstrated the synergistic effect in nucleophilic fluorination when we combined two solvents--ionic liquid (IL) and tert-alcohol--into one molecule. Consequently, these functionalized ILs not only increase the nucleophilic reactivities of the fluoride anion but also remarkably reduce the olefin byproduct. Although the mechanism of this synergistic effect remains to be elucidated, we have illustrated the possibility of solvent engineering for a specific reaction.