Generation of O-1(2) from poly-(3-hexylthiophene) (P3HT) irradiated in the presence of O-2 in chlorobenzene was investigated using ESR measurement, employing 2,2,6,6-tetramethylpiperidine (TEMP) as a spin trap. O-1(2) was found to be formed with O-center dot(2)- being produced in parallel at a branching ratio of 99:1. Additionally, the reaction of P3HT with O-1(2) was investigated using an endoperoxide as the O-1(2) source in CCl4 by MALDI-TOF MS measurement. The upper limit of the rate constant for the reaction of P3HT with O-1(2) was estimated using a kinetics simulation. These findings raise the understanding of the photo-oxidative degradation of P3HT. (C) 2015 Elsevier B.V. All rights reserved.