The thermal reaction of trifluoroiodomethane with vinyl fluoride gives a high yield of a mixture of the two isomeric 1 : 1 adducts 1,1,1,3-tetrafluoro-3-iodopropane and 1,1,1,2-tetrafluoro-3-iodopropane in the ratio 89 : 11. Reaction of trifluoroiodomethane with propene at 200째 gives a high yield of a mixture of the 1 : 1 adducts 1,1,1-trifluoro-3-iodobutane and 1-iodo-2-trifluoromethylpropane in the ratio 90 : 10. Thermal reactions of trifluoroiodomethane at 200째 with the olefins trifluoroethylene, hexafluoropropene, and vinyl chloride give appreciable amounts of 2 : 1 and higher telomers in addition to the 1 : 1 adducts. The extent of telomer formation is much reduced in the case of trifluoroethylene when the reaction is carried out in the presence of added iodine. Competitive reactions of trifluoroiodomethane with mixtures of ethylene and vinyl fluoride or of ethylene and propene show that the reactivity of the olefins towards trifluoromethyl radical attack is in the order: propene > ethylene > vinyl fluoride.