Photolysis, at low temperatures in cyclopropane and/or CFCl 3, of bis[(E)-4,4-dimethylpent-2-enoyl]-peroxide 1, bis(4,4-dimethylpent-2-ynoyl) peroxide 2 and dipropyl peroxydicarbonate 3 yielded the corresponding carbonyloxyl radicals, viz. Me 3CCH=CHC(O)O ., Me 3CC≡CC(O)O . and PrOC(O)O ., which were identified by EPR spectroscopy. Carbonyloxyl radicals were not detected on photolysis of bis(3-methylbut-2-enoyl) peroxide 4, di[(E)-cinnamoyl] peroxide 7, and bis(1-methylpropyl) peroxydicarbonate 9. All the peroxides also give other radicals many of which were identified from their EPR spectral parameters. Thus, the two bis(alkenoyl) peroxides give the 'corresponding' alkenylacyl radicals, viz. s-cis- and s-trans-Me 3CCH=CHĊ=O from 1 and s-trans-Me 2C=CHĊ=O from compound 2. Evidence is presented which indicates that these acyl radicals may be formed directly during the photolysis of substrates 1 and 4 via a Norrish type-1 process. This would represent a new route for the photodecomposition of acyl peroxides with C-O bond scission occurring in competition with the usual O-O bond scission.