Fourier transform infrared (FTIR) spectroscopy was used to investigate the effect of a commercial photoinitiator [2,2-dimethoxy-2-phenylacetophenone (DMPA); Irgacure 651, Ciba, Basel, Switzerland] incorporated into poly(methyl methacrylate) (PMMA). Pure PMMA and modified PMMA (containing 5% DMPA) films were exposed to a xenon lamp in air. Qualitative and quantitative analyses of the recorded FTIR absorption spectra were done. This allowed us to determine the differences in the course of polymer photooxidative degradation occurring in PMMA and PMMA doped with DMPA. We found that the initiator added to polymer film enhanced the efficiency of the formation of the photooxidized products containing functional groups of different types, which could be distinguished by mathematical treatment of the FTIR spectra. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1598-1607, 2010