The microstructure of maleic anhydride (MA) grafted polyethylene (PE), MA-g-PE, has been successfully characterized by high-resolution NMR and FT-IR spectroscopy. The carbonyl asymmetric and symmetric stretchings of MA-g-PE exhibit lower shifts with an increase in the grafting degree of MA, suggesting stronger inductive interactions between different grafts in a solid state. The H-1 and C-13 NMR spectroscopy of [2,3-C-13(2)]MA grafted PE with a low molecular weight (lmPE), [2,3-C-13(2)]MA-g-lmPE, demonstrates that [2,3-C-13(2)]MA-g-lmPE contains fewer succinic anhydride (SA) oligomeric grafts with a terminal unsaturated MA ring (oligo-MA) in addition to more SA oligomeric grafts terminated with a saturated SA ring (oligo-SA), the only structure that had been observed prior to this study. The formation of oligo-MA and oligo-SA can be explained by a mechanism based on two different termination processes: disproportionation between a grafting radical and a macroradical on a secondary carbon in lmPE or another grafting radical as well as hydrogen abstraction of a grafting radical from a secondary carbon on lmPE.