Post-use high-density polyethylene, almost devoid of any stabilizing agents, was restabilized in various degrees and subjected to multiple extrusion cycles at different reprocessing temperatures for assessing its chemical stability. The process-induced material degradation was attributed primarily to long-chain branching caused by crosslinking. It was monitored by an increase in viscosity and evaluated on the basis of an approximate expression derived using fundamental principles of macromolecular rheology. It was determined that long-chain branching increases with temperature and the extent of processing, while decreasing with the amount of restabilizing agent added. A simple model was developed to quantitatively describe the progress of the chemical change by relating it to key material and operational variables. Besides constituting a useful method of monitoring and controlling polymer modification during processing, this model suggests ways of optimizing stabilization according to the particular processing and product requirements.