This article reports a study of the influence of PET photooxidation on the oxygen barrier properties of this polymeric material. It is shown that the photochemically induced oxidation of PET provokes a decrease of the oxygen permeability coefficient, which affects the properties of the material. This effect is shown to result from crosslinking by recombination of macroradicals formed by the photooxidation of PET. The global effect that is observed is thickness-dependent. As a consequence of the attenuation of the UV light that penetrates the sample, photochemical oxidation of PET only occurs in the first 35 microns from the exposed surface, meaning that for samples with a thickness larger than that of the oxidised layer, the permeability decreases at the surface, whereas that of the core of the sample is not modified. This is corroborated by measurements of the variations of permeability coefficients for films with different thicknesses.