Abstract The thermo-oxidative stability of high density polyethylene was studied using multiple extrusions. Unstabilized Phillips HDPE (Cr-catalyst) showed a tendency towards crosslinking whereas unstabilized Ziegler HDPE (Ticatalyst) favoured chain scission. The molecular weight distribution and polydispersity of the polymers changed during processing. Functional group concentral ions were measured by FT1R spectroscopy and related to rheological changes (melt flow, polydispersity, MW). The reactions of the vinyl group are primarily responsible for the molecular weight increase. The increase correlated with the content of the vinyl groups originally present. Trans-vinylene and vinylidene groups did not participate in radical addition reactions. The formation of trans-vinylene groups is a characteristic result of processing HDPE. Phenolic antioxidants and phosphite stabilizers are effective in preventing the reactions of vinyl groups. Blends of phenolic antioxidants and phosphites exhibit synergism.