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 (1O2) could be produced by irradiation of P3HT by photosensitization, with no oxidation of the polymer. In addition, 1O2 was directly formed on P3HT via a chemical reaction, again with no oxidation of the polymer. These results give strong evidence that 1O2 is not the principal photo-oxidative degradation intermediate of P3HT, which conflicts with previous reports.