Alkenyloxy(diphenyl)silanes that contain a terminal double bond undergo radical-chain cyclisation at 60–65 °C, in the presence of di-tert-butyl hyponitrite as initiator and a thiol as a catalyst. The thiol acts as a polarity-reversal catalyst and promotes the overall abstraction of hydrogen from the Si–H group in the alkenyloxysilane by the cyclic carbon-centred radical, formed by intramolecular addition of the corresponding silyl radical to the CCH₂ group. Allyloxysilanes give five-membered-ring products via 5-endo-trig cyclisation of the intermediate allyloxysilyl radical. Homoallyloxysilanes give mixtures of five- and six-membered heterocycles, but the intermediate silyl radicals undergo predominantly 6-endo cyclisation, in contrast to the corresponding carbon-centred radicals which cyclise preferentially in the 5-exo mode. An analogous pentenyloxysilane gives only the seven-membered-ring product via a 7-endo radical cyclisation. Steric effects play an important part in influencing the final-product stereochemistry when this is determined in the hydrogen-atom transfer reaction between the cyclic adduct radical and the thiol catalyst. Complementary EPR spectroscopic studies of the short-lived intermediate cyclic adduct radicals have been carried out in the absence of thiol and the structures and conformations of these species have been determined. It is emphasised that, for thiol catalysis of the overall cyclisation of alkenyloxysilanes to be successful, it is necessary for the addition of the chain-carrying thiyl radical to the CCH₂ group to be reversible under the reaction conditions.