In the present article, the graft copolymerization of 4-vinyl pyridine onto guar gum initiated by potassium peroxymonosulphate /ascorbic acid redox pair in an aqueous medium was studied gravimetrically under a nitrogen atmosphere. Grafting ratio, grafting efficiency, and add on increased on increasing the concentration of potassium peroxymonosulphate from 5.0 x 10(-4) to 10 X 10(-4) mol/L and ascorbic acid concentration from 0.4 x 10(-3) to 2.0 x 10(-3) mol/L. On increasing the hydrogen ion concentration from 2.5 x 10(-3) to 10.0 X 10(-3) mol/L, grafting ratio, efficiency, add on and conversion were increased. Maximum grafting was obtained when guar gum and monomer concentration were 1.0 g/L and 20.0 X 10(-2) mol/L, respectively. An increase in temperature from 30 to 35 degrees C increased the grafting ratio, but conversion and homopolymer decreased. The graft copolymers were characterized by IR spectroscopy and thermogravimetric analysis. (C) 2007 Wiley Periodicals, Inc.