Poly(3-hexylthiophene-2,5-diyl) (P3HT) is an organic semiconducting polymer that is widely used in organic electronic devices such as organic solar cells. In this work, we studied the oxidation behavior of P3HT on glass under exposure to visible light and dry air. We also compared the oxidation behavior of bare P3HT films with that of P3HT/phenyl-C-61-butyric acid methyl ester (PCBM) bulk heterojunction systems consisting of donor and acceptor. UV-Vis absorption spectroscopy and X-ray photoelectron spectroscopy were employed to provide evidence that optically excited P3HT could undergo oxidation, which was likely initiated via electron transfer from the conduction band of P3HT to O-2, forming strongly oxidizing O-2(-) species. By contrast, P3HT + PCBM yielded almost no oxidation of organic materials under the same conditions. PCBM was thus suggested to extract optically excited electrons from P3HT, thereby inhibiting the formation of O-2(-). PCBM thus played a role in separating electron-hole pairs from the photoactive layers of organic solar cells and affected the passivating oxidation of photoactive conjugate polymers.