Graft copolymers of soybean protein isolate (SPI) and methacrylic acid (MAA) were prepared in an 8 mol/L urea aqueous solution with ammonium persulfate (APS) as an initiator, beta-mercaptoethanol as an unfolding agent for SPI, and a chain-transfer agent. Evidence of grafting was obtained by the comparison of the Fourier transform infrared and NMR spectra of SPI with those of the SPI-grafted MAA copolymer [SPI-g-poly(methacrylic acid) (PMAA)]. A possible copolymerization mechanism of SPI and MAA was determined, and the copolymerization rate equation was derived. The effect of beta-mercaptoethanol content, APS content, and reaction temperature on the graft copolymerization was studied by the determination of the grafting parameters, including grafting percentage and grafting efficiency. Dynamic laser light scattering was used to investigate the effect of the pH value on the hydrodynamic radius of SPI and the grafted SPI aggregate in aqueous solution. The average hydrodynamic radius of SPI-g-PMAA aggregate was much smaller than that of the SPI aggregate at about the isoelectric point of SPI and a high pH value, and the hydrodynamic radius distribution of the SPI-g-PMAA aggregate was narrower than that of the SPI aggregate. (c) 2006 Wiley Periodicals, Inc.