Films of polystyrene (PS) were exposed to accelerated weathering in a Xenotest Alpha apparatus to mimic outdoor conditions (λ>=300 nm). The changes of the chemical structure in the films were studied by Fourier transform infrared spectroscopy (FT-IR) and the surface photo-oxidation by X-ray-photoelectron spectroscopy (XPS). The number of main-chain scissions and the changes in the molecular weight distribution were estimated by gel permeation chromatography (GPC). In particular, during the first stage of the photo-oxidation, the mechanical properties (stress at the breaking point (σ b ) and strain at the breaking point (ε b )) of the PS films are strongly dependent on the degree of surface oxidation.