UV photo-oxidation of polystyrene was achieved with 253.7 and 253.7/184.9 nm radiation in the presence of an atmospheric pressure of oxygen and analyzed by X-ray photoelectron spectroscopy as a function of treatment time. A methodical increase in the atomic percentage (at.%) of oxygen occurred during 253.7 nm activation up to 2 h of treatment time. Photo-oxidation with the 253.7/184.9 nm lamps resulted in a saturation level of ca. 35 at.% O. Initially, C-O and C=O groups were formed and then O-C=O type moieties. Poly (acrylic acid) was partially and/or thinly grafted onto the oxidized polystyrene surfaces.