The interaction of poly(3-hexylthiophene), P3HT, with oxygen has been investigated. Evidence-in the form of UV - vis absorption, electron paramagnetic resonance data, and a field effect conductivity analysis of a polymer-based field effect transistor-is presented for the existence of a reversible charge transfer complex between P3HT and oxygen. Oxygen-sensor applications can be foreseen for thin film transistors incorporating P3HT.