The modification of the conductivity as well as the nanoscale morphology and electrostatic properties of poly-3-octyl-thiophene (P3OT) thin films have been studied as these films are irradiated by ultraviolet light (UV). Films of about 100 nm thickness were prepared by spin-coating a P3OT solution in toluene on glass substrates. The samples were characterized by electronic transport measurements and non-contact scanning force microscopy (NC-SFM) for every cycle of UV radiation. Nanoscale topographic and electrostatic characterization using NC-SFM were performed on the same location using a specially designed sample holder. A two stage degradation process has been observed, the first one presents a chemical modification of the polymer (decoloration of sample) and free carrier mobility reduction, the second one is characterized by a strong structural modification, thickness reduction, oxygen doping and further mobility reduction. The comparison of these results on P3OT with other studies performed on P3HT will allow for a detailed analysis of the role of side-chains in the degradation mechanism upon UV irradiation. (C) 2010 Elsevier B.V. All rights reserved.