Commercial initiator – 2,2-dimethoxy-2-phenyl acetophenone (Irgacure 651, Ciba, Switzerland) was introduced to poly(methyl methacrylate) (PMMA) matrix to enhance the polymer degradation. Two types of thin polymeric films (PMMA and PMMA+5% Irgacure) were UV-irradiated in laboratory conditions (Xenon lamp, air atmosphere, room temperature). The photochemical changes in pure PMMA and PMMA doped with the initiator were studied by absorption spectroscopy in the UV-Vis range. Separately, the behaviour of the initiator in the acetonitrile (ACN) solution upon UV-radiation was investigated at the same conditions. It was found that the addition of the Irgacure 651 to PMMA accelerated the photooxidative degradation, particularly at the early stages of exposure. The efficiency of formation of chromophoric groups in PMMA in the presence of the Irgacure initiator was significantly greater than in pure polymer. This suggests that the free-radical products of the initiator photolysis (mainly benzoyl radicals) are able to abstract hydrogen atoms from PMMA molecules.