Raman spectroscopy is employed as a structural probe of both oxygen-free and oxygenated thin films of Cso. Comparison of initial spectra reveals no influence of oxygen inclusion on the Raman frequencies. The amplitudes of the peaks are, however, observed to be sensitive to light exposure. Each of the initially observed peaks degrades on a time scale of tens of seconds while new peaks emerge approximately ten wavenumbers to the low-energy side. In oxygenated films the degradation process is apparently slower. The process is analyzed using a simple rate equation model of a level scheme in which population of the triplet state leads to a sample degradation with a characteristic time rZ, competing with the ground state recovery, characterized by the triplet lifetime, 7,. The decay of all the Raman peaks in oxygen-free films fits well to such a model with values of 80f 5 s and 55 f 5 ms for rZ and r,, respectively. The rise of the new spectral features, corresponding to the degraded product, also tits the model. In oxygenated films, where a similar value of TZ is found, the best tits yield a triplet lifetime of 30+ 5 ms. It is proposed that the role of oxygen is to inhibit the photochemical degradation via triplet quenching.