Communication: The operating lifetime of light-emitting devices (LEDs) based on poly(p-phenylenevinylene) (PPV), one of the most widely used conjugated polymers for polymer-based LEDs, is sensitive to air exposure. The effects associated with the photo-oxidation of PPV in air are examined. Possible photo-induced oxidation products are proposed based on simulations, using stilbene as a model compound. X-ray photoelectron spectroscopy, ultraviolet photo-electron spectroscopy, and quantum-chemical calculations all indicate that a series of photo-oxidation products form on the surface of the PPV thin films.