The heterogeneous free-radical grafting of maleic anhydride (MAH) onto poly(4-methyl-1-pentene) (PMP) was studied using supercritical carbon dioxide as the solvent/swellant. The anhydridecontent in the product can be controlled by using variations in time and temperature to achieve maleationsas high as 7 mol % at 125 °C after a 5 h reaction period. A comparison between benzoyl peroxide (BPO)and dicumyl peroxide (DCP) initiators indicates that DCP is the more effective initiator. The effect ofreactant concentrations on the degree of grafting is also examined, yielding an optimum MAH:DCP molarratio of 3:2. Competitive cross-linking of the substrate is the major drawback to this means of polymermodification.