P3HT layers with a thickness of ∼5 nm were deposited on bare and TiO2-covered ZnO ripple structures. The ZnO ripples were prepared wet-chemically and a TiO2 layer with a thickness less than 5 nm was prepared by atomic layer deposition. Under humid air and visible light illumination, the oxidation behaviors of P3HT on these surfaces were studied using photoelectron spectroscopy. It was found that P3HT on TiO2/ZnO oxidizes more easily than that on bare ZnO ripples. Using a model substrate of a flat ZnO surface in combination with angle-resolved photoelectron spectroscopy, we found that oxidation of P3HT occurs at the surface of the topmost layer of P3HT, not at the P3HT/oxide interfaces, even though P3HT oxidation is strongly influenced by the interface structure. It is suggested that the lifetime of electron-hole pairs can be strongly influenced by the interface structure, which can also affect the oxidation behavior of P3HT.