The graft copolymerization of three vinyl monomer species, acrylonitrile (AN), acrylamide (AAm), and acrylic acid (AA), onto starch was carried out with eerie salt (Ce salt) as an initiator. With 3 mmol/L Ce salt, the monomer activity onto starch decreased in the following order: AN > AAm > AA. Grafting efficiency with AN as the grafting monomer was greater than 90%, but with AA and AAm, it was less than 50%. Starch graft-polyacrylonitrile was hydrolyzed to introduce amide and carboxyl groups into starch. The hydrolyzates were analyzed with infrared spectroscopy. The hydrolysis reaction was accelerated with increasing alkali concentration, reaction temperature, and time. The water absorbancy of the hydrolyzate increased with an increasing carboxyl molar fraction in the polymer, and it dissolved in water above a 0.6 molar fraction. The absorbancy of water was 2 times higher than that of a NaCl aqueous solution. The copper-ion-exchange capacity of the sample was greater in graft copolymers with higher carboxyl group contents.