Reactive extrusion was employed to graft itaconic anhydride (IA) onto polyethylene, using thermally induced peroxide decomposition. It was found that an increase in IA concentration lead to an increase in the degree of grafting (DOG), but only up to 6 wt % IA. Using di-cumyl peroxide (DCP) as the initiator resulted in a higher DOG compared to di-tert-butyl peroxide (DTBP) and required less reaction time to achieve the same DOG. However, raising the IA concentration also resulted in an increase in cross-linking. Increasing the initiator concentration from 0.2 to 2 wt % resulted in a higher DOG. However, 5 wt To initiator showed similar results compared to using 0.2 wt % due to termination by disproportionation, which has been shown to be more prevalent at high initiator concentrations. Degradation was clearly observed by the inability to form a continuous extrudate during extrusion as well as discolouration. A residence time of more than 50 seconds, using DCP and 120 s for DTBP didn't offer any further increase in the DOG and also resulted in more pronounced degradation. Optimizing grafting is therefore a trade-off between maximal DOG and minimizing side reactions. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3118-3126, 2010