Fluoroiodomethane has been photolysed in the gas phase in the presence of ethylene and fluoroethylenes. The main products of these reactions are adducts from the addition of fluoroiodomethyl radicals to the alkene. Experiments using carbon dioxide as an inert diluent or di-t-butyl peroxide as a thermal initiator indicate that excited fluoroiodomethyl radicals are important in the photolytic runs. Only with the two most reactive olefins, ethylene and tetrafluoroethylene, was a chain process sustained at any but the highest temperature. Orientation data and relative rates have been estimated for the addition of fluoromethyl radicals to vinyl fluoride, 1,1-difluoroethylene, and trifluoroethylene at 164°. The relative rate of fluoromethyl radical addition to ethylene and tetrafluoroethylene has been compared over a wide temperature range and the relative Arrhenius parameters [A_{CF2= CF2}/A_{CH2= CH2}= 0.76 ± 0.2; E_{CF2= CF2}–E_{CH2= CH2}=–1.3 ± 0.3 kcal mol^–1] are compared with similar data for methyl, difluoromethyl, and trifluoromethyl radicals. The activation energy difference shows a continuous decrease from trifluoromethyl to methyl radicals.