This report concerns oxidation mechanisms of undoped poly(3-octylthiophene) (POT). Thermal oxidation of POT films has been carried out under conditions relevant to processing. The films have also been exposed to accelerated aging. Samples were heated in air or nitrogen atmospheres at temperatures ranging from 180 to 220 degrees C. The molecular weight changes with oxidation time were studied by size exclusion chromatography (SEC). An increase of molecular weight occurred at high temperatures, which, in the presence of air, led to the formation of insoluble gel. Carbon-13 NMR and infrared spectroscopy were used for the identification of the key products and showed that the major oxygen-containing group is ketone in the alpha-carbon position of the alkyl side chain. Hence, the mechanism for thermal oxidation at processing temperatures indicated mainly a side chain degradation. Photooxidation by UV light of short wavelengths gave five new modes between 1080 and 1330 cm(-1) and a different carbonyl pattern when compared to thermooxidation. These new modes have the same shifts as the doping-induced bands of POT. Thus, photooxidation indicated a chain scission or a ring-opening mechanism.