The rheometer cavity has been used to analyse, in carefully controlled exposure conditions, the macromolecular changes generated by the mechanical recycling of PET during its extrusion. Isothermal ageing tests at 280 °C, under constant or variable oxygen partial pressures (between 0% and 21% of the atmospheric pressure), have allowed to establish that two types of oxidative macromolecular changes take place successively in an extruder reactor. Chain scissions predominate in the "highly oxygenated" zones (at the feeder and die), whereas chain welding predominates in "poorly oxygenated" zones (in the middle) of reactor. Thus, it appears that the relative predominance of both types of modifications is sharply linked to the extruder geometry and size (in particular, to the feeder and die diameter, and screw length). A kinetic model of thermal ageing of molten PET has been built to check these assumptions. It describes satisfyingly all the rheometric results obtained in the present study.