A new technique for preparing freely suspended submicron films of poly(3-nonylthiophene) (PNT) has been developed. These films were characterized by both i.r. and u.v. spectroscopy, allowing us to qualitatively analyse the effects of photo-oxidation and how these effects change with the period of time after the end of the irradiation treatment. Before these spectroscopic measurements were made, PNT was compensated with hydrazine in order to remove any chemical impurities resulting from the polymerization catalyst. The i.r. experiments indicate that when PNT was irradiated by light of wavelength longer than 466 nm, electrons were transferred from the PNT to oxygen. Thus, the photo-oxidation process resulted in the formation of positive polarons on the polymer chains, plus superoxide anions. In the dark, electrons were transferred back to the polymer chains, which therefore returned to their neutral state. This mechanism was supported by e.s.r. experiments. Irradiation by light of wavelength shorter than 466 nm induced i.r. absorptions due to both polarons and sulfonyl groups, as well as conjugation-breaking reactions. When impurities were not removed, the formation of polarons and sulfonyl groups was observed even if wavelengths shorter than 466 nm were excluded. These impurities also accelerated the conjugation-breaking reactions.