The grafting of crotonic acid onto acrylonitrile-butadiene-styrene terpolymer (ABS) was initiated by benzoyl peroxide (BPO) in 1,2-dichloroethane solution. The infrared spectra confirmed that crotonic acid was successfully grafted onto ABS backbone. The influences of the concentration of crotonic acid, BPO, and ABS on grafting were studied. The graft occurred at the butadiene region of ABS. A reaction mechanism was proposed: the grafting most likely took place by cross-termination of poly(crotonic acid) radicals with ABS macroradicals. The initial rate of grafting could be written as: R-p = 2.02 x 10(-4) [P] [M](2) [I-2](0.5)/([M] + 1.05[P])(2).