Abstract. An investigation was made of grafting trans-ethylene-1,2-dicarboxylic acid (TEDA) onto metallocene-linear low-density polyethylene (mLLDPE) and low-density polyethylene (LDPE) in the course of reactive extrusion. The initiator was 1,3-bis-(tert-butyl-peroxyisopropyl)benzene. The graft efficiency of TEDA has been shown to increase with increasing initiator concentration, irrespective of polyethylene type. The graft values for LDPE were higher than for mLLDPE over the initiator concentration range (0.05 to 0.4 wt%). The rheological properties of mLLDPE were found to undergo more tangible changes during functionalization than those of LDPE. These changes were caused by side reactions, mainly macromolecular crosslinking. It has been established that some carboxyl groups get transformed to anhydride groups in the grafted product. The concentration of end double bonds reduces, but intramolecular unsaturation in both polyethylenes increases. Data are presented on thermal and stress-strain (mechanical) properties of virgin and functionalized polymers, as well as rheological and viscoelastic properties of their melts.