Films of regioregular poly(3-hexylthiophene) [P3HT] were monitored for degradation by means of in situ fluorescence quenching measurements. The films were found to be stable under UV or VIS energy illumination in vacuo and stable to the air when kept in the dark. Under illumination in air however, degradation products were formed immediately as seen by the fluorescence quenching. The quantum yield for defect product formation was calculated to be around 3 × 10−6 per incident photon of above band-gap energy, a relatively rapid reaction that could create over 1016 defects per cm3 in optically thin films of P3HT in less than a second under sunlight, or in only a matter of minutes under room lighting. The degradation pathway is thought to involve attack of ground state molecular oxygen on the P3HT excited state. The reaction itself is self-inhibiting, as the P3HT excited state is thought to be effectively quenched by the increasing formation of defects, leading to slower and slower defect formation until the film enters a ‘quasi-stable’ state where no further degradation by this mechanism will occur. The upper limit to the number of defects to be found in films of P3HT formed by this mechanism is estimated to be around 2 × 1017 cm−3.