This paper is devoted to the investigation of the phototransformation mechanisms of PVK exposed to long-wavelength radiations (ì > 300 nm) in the absence and in the presence of oxygen. PVK irradiated as thin films was analyzed by fluorescence, UV-vis, and IR spectroscopies. The photoproducts formed were identified by postirradiation treatments including chemical derivatization reactions and HPLC analysis. The cross-linking was evaluated by gel fraction measurements. PVK is known to give an intense excimer fluorescence and no monomer fluorescence. The phototransformations provoked by light absorption are shown to lead to a dramatic decrease of this fluorescence which can be correlated with the modification of the chemical structure of the macromolecules. The decrease of fluorescence is the result of the diminution of the local mobility of the macromolecular chains resulting from cross-linking reactions provoked by the recombination of the several radicalar species. A general mechanism accounting for the phototransformation of PVK in the presence and in the absence of oxygen is proposed.