Photo-induced graft copolymerization of acrylonitrile (AN) onto sodium salt of partially carboxymethylated guar gum (Na-PCMGG, = 0.291) was carried out in an aqueous medium using ceric ammonium nitrate (CAN) as photo-initiator to synthesize a novel graft copolymer, Na-PCMGG-g-PAN, which may find potential application as a superabsorbent hydrogel. Studying the influence of concentrations of photo-initiator (CAN), nitric acid, monomer (AN) as well as reaction time, temperature, and amount of substrate on the grafting yields, the reaction conditions for optimum grafting were evaluated. Maximum values of the grafting yields achieved were %G = 285.77 and %GE = 70.76 at optimized conditions. The proposed kinetic scheme could explain very well the experimental results. The influence of different kind of acids on the grafting yields was also studied. Grafting process was confirmed with the help of FTIR, thermal (TGA/DSC), and SEM techniques. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41371.