Degradation behaviors of poly(3-hexylthiophene-2,5-diyl) (P3HT) layers on NiO in the presence of H2O at ambient pressure and dark conditions were studied using X-ray photoelectron spectroscopy (XPS). Upon H2O exposure at 120 °C, partial oxidation of P3HT together with molecular water incorporation, but with the maintained local ring-structure, were deduced by XPS. Valence band spectra of XPS evidenced that the partial oxidation of P3HT local structure could alter π-conjugation systems of P3HT layers, forming additional electronic states close to its original highest occupied molecular orbital. For comparison, P3HT surface was also exposed to O2, and no change in the S 2p and C 1s spectra was found by O2 exposure at 120 °C, implying that H2O plays a major role at the initial stage of P3HT oxidation. Copyright © 2014 John Wiley & Sons, Ltd.