Molecular oxygen forms a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes). The complex is weakly bound but possesses a distinct absorption band in the visible region. The electronic properties of the neutral polymer are modulated by the CTC. The extent of modulation of these properties is proportional to oxygen pressure and is fully reversible. Analysis of poly(3-hexylthiophene) field effect transistors under increasing pressures of oxygen shows that the carrier concentration increases, conductivity increases, and the charge carrier mobility is lowered by the formation of the charge transfer complex. The CTC manifests itself as a fluorescence quencher of mobile polaronic excitons, and it explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model. Implications of the charge transfer complex on the photochemistry of poly(3-alkylthiophenes) and photosensitization of singlet oxygen are discussed.