The scope of the hydrosilylation of olefins catalysed by tert-butyl peroxide under UV irradiation has been studied briefly. Some conclusions are as follows: (i) The reaction between trichlorosilane and 1-hexene is not affected by temperatured change in the range -20 to +40°; (ii) good yields of trichlorosilane adduct can be obtained from trichlorosilane and 1-hexene, 1-octene and cyclohexene, but yields are very poor with 2-octene and 2-methyl-2-butene; (iii) dichloro(methyl)silane gives, acceptable yields though markedly lower than those from trichlorosilane, but triethysilane, triethoxysilane, triphenylsilane and 1,1,1,3,5,5,5-heptamethyltrisiloxane do not give detectable yields of adducts under the conditions used. Trichlorosilane has been found to react with benzene under similar conditions to give phenyltrichlorosilane in 16% yield.