A new competitive kinetic method to obtain the relative rates of intramolecular hydrogen abstraction versus intermolecular deuterium abstraction has been developed and used to measure the rates of the relatively slow unimolecular 1,5-hydrogen abstractions by 1,1,2,2-tetrafluoroalkyl (XCH2CH2CH2CF2CF2*) and 1,1,2,2,3,3-hexafluoroalkyl (XCH2CH2CF2CF2CF2*) radicals. Substituents at the site of C-H abstraction (X = CH3O-, CH3S-, CH3-; C6H5-, CH3C(O)O-) have only a moderate effect, with the rate constants for 1,5-shifts in the 10(3) s(-1) range. Values of k(H) and k(D) for bimolecular hydrogen/deuterium transfer from t-BuMe2SiH(D) and Me3SiSiMe2H(D) are reported along with measures of side-chain H transfer from the deuterated silanes.