The graft copolymerization reaction of acrylamide onto guar gum with a vanadium(V)/mandelic acid redox pair was carried out in an N-2 atmosphere. The optimum concentrations of vanadium(V), mandelic acid, hydrogen ions, acrylamide, and guar gum for the maximum percentage of grafting were 6.0 X 10(-3), 2.0 x 10(-2), 55.0 x 10(-2), and 20.0 x 10(-2) mol/dm(3) and 110.1 X 10(-2) g/dm(3), respectively. The optimum time and temperature of reaction were 90 min and 35 degrees C, respectively, and during the study of [H+] variation, a prompt change in the value of the grafting parameters was observed. The maximum percentage of swelling of the graft copolymer was achieved at room temperature in 1 h. Studies of the flocculation, viscosity, and metal-ion absorption capacity were also performed. The synthesized graft copolymer was characterized by Fourier transform infrared spectroscopy and thermogravimetric analyses, which showed that the grafted guar gum was thermally more stable than the ungrafted guar gum. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2375-2385, 2010