Steric exclusion chromatography (SEC) and rheometry were used to study irreversible structural changes occurring during poly(ethylene terephthalate) (PET) extrusion. It appeared that polymer gelation could occur after three successive extrusions, showing the existence of a crosslinking process. This latter occurs only in the presence of oxygen at low oxygen pressures (typically P-O2 < 9 x 10(3) Pa) as a result of coupling of alkyl radicals generated by the oxidation chain process. A kinetic model based on this assumption but taking also into account chemical reactions occurring in nitrogen, generates kinetic curves of melt (Newtonian) viscosity changes in good agreement with experimental ones. (C) 2004 Elsevier Ltd. All rights reserved.