Oxidative changes in poly [methyl(phenyl)silanediyl] (PMPSi) films exposed in a Weather-O-Meter ATLAS Ci 3000+ were monitored by Fourier transform infrared spectroscopy. Increases in absorption in the siloxane, carbonyl, and hydroxyl regions, decreases in absorption in the Si-Si region, and changes in the aromatic C-H bond region were monitored. Experiments in argon and air indicated the key role of oxygen in the process. The photooxidation of PMPSi was retarded by triazine- and oxalanilide-based ultraviolet absorbers [2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyloxy)phenol and N-(4-tert-butyl-2-ethoxyphenyl)-N'-(2-ethylphenyl)oxalanilide] protecting the polymer by the excited-state intramolecular proton-transfer mechanism. (C) 2003 Wiley Periodicals, Inc.