Soy protein concentrate was grafted with ethylmethacrylate using ascorbic acid/potassium persulphate as a redox initiator. Different reaction parameters, such as reaction time, reaction temperature, solvent amount, initiator ratio, and pH, and monomer concentration were optimized to get maximum graft yield (134.12%). The graft copolymer formed was characterized by Fourier transform infrared spectrophotometer, X-ray diffraction, and scanning electron microscope techniques. Thermogravimetric analysis, differential thermal analysis, and differential thermogravimetric analysis studies showed that graft copolymer was thermally more stable than the backbone. Thermal decomposition studies indicated that the rate of weight loss per minute was found more in case of backbone when compared with that of graft copolymer. Further, graft copolymer was also found more resistant toward acid-base attack and was found to be more water repellent. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2183-2190, 2011