The degradation of polypropylene-polyethylene copolymers (which were used to make the seats for the Olympic stadium in Barcelona, Spain), in response to natural aging for 2.5 years and artificial aging for 5000 h in a xenon lamp chamber, was studied. The extent of the various photoreactions, including the formation of carbonyl groups, the scission of hydrocarbonated chains, the formation of free radicals in tertiary carbons, and the formation of nonsaturated bonds followed by branching and crosslinking, was analyzed by Fourier transform infrared spectrophotometry. Melting and thermal decomposition were studied by differential scanning calorimetry. In all cases, aging led to a loss crystallinity. By scanning electron microscopy, it was possible to detect the formation of a porous material after melting crystallization, as well as the presence of micelle orientation. Finally, a comparison between natural and artificial aging effects showed differences, the samples exposed to weathering being modified to a greater extent than the artificially aged ones.