The photooxidative degradation of poly-3-hexylthiophene (P3HT) in chlorobenzene solution containing dissolved oxygen was performed at room temperature. The generation of superoxide radical anion (O-2(center dot-)) during the photooxidation of P3HT was verified by ESR measurements using 5,5-dimethyl-1-pyrro-line-N-oxide (DMPO) as the spin trap. The changes in mass spectra determined by MALDI-TOF MS showed that P3HT photodegradation was prevented in the presence of DMPO. Kinetics analysis shows that the prevention of P3HT photodegradation is due to the scavenging of O-2(center dot-) by DMPO. Therefore, the involvement of O-2(center dot-) in the initial step of P3HT photodegradation could be concluded. (C) 2014 Elsevier B.V. All rights reserved.