Graft copolymerization of acrylic acid (AAc) onto polystyrene (PS) by the solid phase graft copolymerization technique was studied. Benzoyl peroxide (BPO) was used as the free radical initiator. Acrylic acid was introduced into the reactor as an aqueous solution. Optimum reaction conditions to obtain maximum grafting were evaluated. The variables studied were temperature, initiator concentration, monomer concentration, acrylic acid-to-water ratio, and reaction time. The graft copolymer was characterized by infrared (IR) spectroscopy, and the graft level was quantified by the IR absorbance ratio technique and titration.