Résumé : Decrease of the intensity of excimer fluorescence during the photo-oxidation of polystyrene indicates that energy transfer occurs from excited phenyl chromophores to oxidation products. Among these, hydroperoxide groups are prominent because, after acting as energy acceptors, they decompose efficiently into free radicals which initiate the photo-oxidation. This effect is clearly demonstrated using cumene hydroperoxide as a model compound for polystyrene hydroperoxides. It is concluded that the sensitized decomposition of hydroperoxide groups provides a general explanation for the characteristic kinetic behaviour of polymers in photo-oxidation as opposed to autoxidation. © 1978.