The graft copolymerization of maleic anhydride (MAH) onto a acryloni-trile-butadiene-styrene (ABS) terpolymer was carried out using benzoyl peroxide (BPO) as an initiator and toluene as a solvent. The effects of various parameters such as monomer concentration, initiator concentration, reaction time, and temperature on graft yield were studied. Addition occurs in the butadiene region of the polymer, either by the loss of vinylic hydrogen and subsequent radical formation and addition of monomer or by addition to the double bond. The graft copolymers were characterized by infrared, thermogravirnetric analysis, and differential scanning calorimetry. Thermal stability was improved andTgwas increased.