The authors follow in real time and under controlled conditions the oxidation of the organic semiconductor rubrene grown on Si O ₂ using spectroscopic ellipsometry. They derive the complex dielectric function ε₁+iε₂ for pristine and oxidized rubrene showing that the oxidation is accompanied by a significant change of the optical properties, namely, the absorption. The authors observe that photo-oxidation of rubrene is orders of magnitude faster than oxidation without illumination. By following different absorption bands (around 2.5 and 4.0 eV for pristine rubrene and around 4.9 eV for oxidized rubrene) they infer that the observed photo-oxidation of these films involves non-Fickian diffusion mechanisms.