A new method for introducing IF and BrF into organic compounds is developed. These reagents have been prepared by the direct action of elemental fluorine on the two respective halogens at low temperatures. The reactions of these interhalogen compounds with isolated double bonds are fast even at -75 °C. They were found to be stereospecific (anti mode addition) and in most cases regiospecific too. With conjugated enones the reactions were considerably slower. In this case an easy HF elimination takes place, producing eventually a-halo enones. Since the benzylic C-I bond is very weak, the reaction of IF with stilbenes leads to the 1,2-difluoro adducts through a secondary attack of the nucleophilic fluorine on the iodine, either through SN2 or through the rare SNi mechanism. Bromine fluoride on the other hand gave the expected BrF adducts. The BrF reactions are much more forceful than the parallel IF ones, and a proton donor has to be present in order to tame this reagent. The halogen fluoride adducts can serve as an entry for synthesis of other fluorine-containing compounds that are difficult to prepare. Thus both the CF2 group and the rare β-fluoro ether moiety were easily obtained from various olefins.