Photoageing reactions and kinetics on long wavelength irradiation of an electron beam (EB) cured resin are presented. The material studied is an aliphatic polyurethane acrylate resin. The chemical photoreactivity of the polymer was followed by infrared spectroscopy and UV-visible spectrophotometry. Two periods of photo-oxidation were found. In the first stage the reaction of acrylate functions remaining after EB polymerization leads to additional crosslinking and to oxidized products. The tip of an atomic force microscope was used as an indenter to follow the variation of stiffness at the surface resulting from crosslinking. In the second stage, oxidation rate decreases and photodegradation products are formed at a constant rate. A heterogeneous distribution of residual acrylates was evidenced across the coating thickness (thick films of 100 μm). An oxidation profile was also observed in the depth of the film after photoageing.