The first two chapters of this thesis describe a study of the kinetics of the gas phase addition of perfluoro-isopropyl and perfluoro-t-butyl radicals to various olefins. The radicals were produced by photolysing the corresponding perfluoroalkyl iodides. Temperature variation data allowed determination of relative Arrhenius parameters. The results are compared with the data available for other simple radicals, and a comparison between the radicals of the series CF3•, CF3CF2•, (CF3)2CF• and (CF3)3C •, in particular, gives valuable information as to the relative importance of steric and other effects in free radical addition reactions. Chapter 3 describes an attempt to determine the absolute rate constant for the recombination of perfluoro- isopropyl radicals. The rotating sector method was used, but, due to a certain amount of thermal initiation and to a first order termination process as well as the more usual bimolecular disappearance of the radicals, the simple mathematical treatment, presented by Shepp, could not be applied to our system. A modified treatment was used and the rate constant was determined at 46°c. The result was taken as evidence for an activation energy different from zero for the reaction. Chapter 4 is a study of the orientation of the addition of monoiodomethyl radicals to various fluorinated olefins, at 150°c Di-t-butyl peroxide was thermally decomposed and the methyl radicals produced initiated the free radical addition by astracting ioding for diiodomethane. The results are compared with the date of some related radicals.