The degradation of different polyethylenes-low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE)-with and without antioxidants and at different oxygen concentrations in the polymer granulates, have been studied in extrusion coating processing. The degradation was followed by online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography-mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto-oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice, and the quenching point is mainly related to the exposure time to air oxygen. Melt temperatures above 280degreesC give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching too. A number of degradation products were identified-for example, alclehydes and organic acids-which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7much greater thanC2 for LDPE, C5>C6>C4> C7much greater thanC2 for LLDPE, and C5>C6>C7>C4much greater thanC2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. (C) 2003 Wiley Periodicals, Inc. J Appl Polyrn Sci 91: 1525-1537, 2004.