Mono- and difluorinations of sulfides were achieved using a novel fluorinating reagent, IF5–Et3N–3HF. The reagent is applicable for substrates having various electron-withdrawing groups, such as an ester, amide, ketone, nitrile, sulfone, or trifluoromethyl. Because more than one fluorine atom on IF5 is used, a large excess amount of the reagent is not necessary, even for the difluorination reaction.