The graft copolymerization of maleic anhydride (MAH) onto acrylonitrile-butadiene-styrene terpolymer (ABS) was carried out with dicumyl peroxide (DCP) and benzoyl peroxide (BPO) as the binary initiators and with styrene as the comonomer in the molten state. IR spectra confirmed that MAH was successfully grafted onto the ABS backbone. A reaction mechanism was proposed: the grafting most likely took place through the addition of MAH radicals to the double bond of the butadiene region of ABS. Influences such as the MAH concentration, the initiators and their concentrations, the reaction temperature, the rotating speed, and the comonomer concentration were studied. The results indicated that using styrene as a comonomer and DCP/BPO as binary initiators was beneficial for the graft copolymerization. (C) 2003 Wiley Periodicals, Inc.