Roles of singlet oxygen (O-1(2)) in photo-induced degradation of poly(3-hexylthiophene) (P3HT) and poly(3-hexyloxythiophene) (P3HOT) were examined. The results indicate that O-1(2) has a considerable role in the degradation of P3HT and P3HOT. Photo-irradiation of P3HT in air resulted in the reduction of the pi-conjugation. The photo-induced reduction also occurred for P3HOT, and it was much faster than that of P3HT. Manceau et al. have reported that O-1(2) was generated by photo-irradiation of P3HT in the presence of oxygen and, nevertheless, O-1(2) is not the principal photo-oxidative degradation intermediate of P3HT (Manceau et al., Macromol Rapid Commun 29:1823-1827, 2008). However, exposures of solid P3HT and P3HOT to chemically generated O-1(2) provided decreases in the pi pi*absorption in our study. The decrease for P3HOT was much larger than that for P3HT. The exposure of P3HOT showed the formation of polarons. The difference in P3HT and P3HOT for the effects of the photo-irradiation and of O-1(2) will be ascribed to the difference in their ionization potentials.