The effects on macromolecular structure brought about by accelerated ageing in linear and crosslinked polyethylenes were detected and quantitatively monitored by using the ATR-FTIR technique. The thermo-processing conditions used in the preparation of samples with various crosslinking levels lead to the formation of by products such as acetophenone and cumyl alcohol, and new groups such as hydroxyl, vinyl and carbonyl in the polymer chain. The main chemical modifications were carbonyl formation of various kinds, which were identified and quantified in the ATR-FTIR spectra of degraded polyethylene samples. The ATR-FTIR methodology provided a quantification of each carbonyl type/kg polyethylene, without any previous sample treatment, thus allowing a real evaluation of the effects of exposure conditions on the materials.