We studied the role that singlet oxygen plays in the solid-state photochemistry of poly(3-hexylthiophene) (P3HT). The photosensitized formation of singlet oxygen by solid-state P3HT and its subsequent reactivity on the polymer were investigated. Using a fluorescent probe, it was found that singlet oxygen(O-1(2))could be produced by irradiation of P3HT by photosensitization, with no oxidation of the polymer. In addition, O-1(2) was directly formed on P3HT via a chemical reaction, again with no oxidation of the polymer. These results give strong evidence that O-1(2) is not the principal photo-oxidative degradation intermediate of P3HT, which conflicts with previous reports.