Polyacrylonitrile (PAN) films were exposed to radiations of long wavelengths (λ ≥ 300 nm) and of short wavelength (λ = 254 nm) under atmospheric oxygen. Changes in the Fourier-transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectra were followed and compared to those obtained during the thermolysis and the thermooxidation of PAN films at 150°C. The products formed in these four conditions of degradation were identified by coupling FTIR and UV-Vis analysis to chemical derivatisation reactions and physical treatments. An analysis of the gas phase has also been done. A general mechanism accounting for thermolysis, thermooxidation and photooxidation is proposed.