The objective of this contribution is to study the effect of compatibilization and additivation on PET/HDPE blends by torque rheometry. Mixtures with virgin and recycled PET and HDPE in proportions 1:3, 1:1 and 3:1 were compounded in a laboratory internal mixer. Pristine blends, as well as blends compatibilized with maleated polyethylene, additivated with Joncryl (multifunctional epoxydic oligomer), and simultaneously compatibilized and additivated were tested in terms of torque and rate of torque variation with time in last stage of melt processing. Both classic compatibilization (PE-g-MA) and addition of Joncryl resulted in significant increases in viscosity, and the combination of compatibilization/additivation showed a synergistic effect. The effect was particularly strong in the blends prepared with recycled PET. Compatiblization and additivation have a moderate protective effect on incipient degradation during processing, particularly on additivated PET-rich blends, attributed to chain extension activity of Joncryl on the polyester.