The gas phase reaction of CH2Cll with ethylene and vinyl fluoride, initiated photochemically and thermally with di-t-butyl peroxide, has been investigated, and the relative rates of addition of chloromethyl radicals to both ends of vinyl fluoride have been determined over a range of temperatures. The mechanism of the reaction is discussed, and the relative Arrhenius parameters for addition to the two alkenes are interpreted in terms of frontier orbital coefficients.