The formation of the rearrangement ketone 3-phenyl-1-(phenylmethoxy)-2-propanone (3) in the thermolysis of 3,3-bis(phenylmethyl)-1,2-dioxetane (1) was rigorously probed for radical-induced chain decomposition. The latter mechanism appears to operate on the basis of the following experimental facts : (a) thermolysis of a mixture of the dioxetanes 1-h(4) and 1-d(4) afforded significant amounts of the crossover beta-keto ethers 3-h(2) and 3-d(2), (b) decomposition in toluene-d(8) in the presence of tert-butyl peroxalate (4) resulted in substantial incorporation of externally generated phenylmethyl-d(7) radicals in the form of the a-keto ether 3-d(7), and (c) in the presence of the triplet quenchers dibromomethane and 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) the formation of the beta-keto ether 3 was suppressed. Clearly, in the thermolysis of dioxetane 1, free phenylmethyl radicals are generated through Norrish type I cleavage of triplet-excited 1,3-diphenyl-2-propanone (2). Attack on the dioxetane peroxide bond leads to the beta-keto ether 3 by beta cleavage of the resulting alkoxy radical with regeneration of the chain-carrying phenylmethyl radical.