The grafting of styrene on soy protein isolate (SPI) in an 8 moL/L urea aqueous solution initiated by ammonium cerous nitrate and potassium persulfate was studied. The grafted copolymers were characterized by IR spectroscopy and DSC. The results indicated that styrene was grafted on the SPI. The influence of the reaction conditions on the grafting and efficiency percentages was investigated. The grafting and efficiency percentages initially increased and then decreased with the increase of the initiator concentration, monomer concentration, and reaction temperature. With the increase of reaction time, the grafting and efficiency percentages increased. (c) 2005 Wiley Periodicals, Inc.