Graft copolymerization of methacrylic acid (MAA) onto guar gum (GG) was carried out by free radical initiation mechanism by using potassium persulfate (PPS) as an initiator. It was found that % grafting, grafting efficiency, and % conversion were all dependent on the concentration of PPS, MAA, reaction temperature, and reaction time. Using PPS, the maximum % grafting was ascertained to be 241 at the optimum conditions of 60 degrees C reaction temperature, 3 h of reaction time, 1.1 mmol of PPS, and 0.058 mol of MAA. Plausible mechanism for grafting reaction was suggested. The graft copolymer formed was characterized by Fourier transform infrared and differential scanning calorimetry. The graft copolymer formed could find applications in drug delivery. (c) 2006 Wiley Periodicals, Inc.