Novel types of highly swelling hydrogels were prepared by grafting crosslinked polyacrylic acid-co-poly-2-acrylamido-2-methylpropanesulfonic acid (PAA-co-PAMPS) chains onto kappa-carrageenan through a free radical polymerization method. Here, we propose a mechanism for k-carrageenan-g-PAA-co-PAMPS formation and confirm the hydrogel structure using FTIR spectroscopy. The effect of grafting variables (i.e. concentration of methylenebisacrylamide (MBA), acrylic acid/-2-acrylamido-2-methylpropanesulfonic acid (AA/AMPS) weight ratio, ammonium persulfate (APS), kappa-carrageenan, neutralization percent and reaction temperature) were systematically optimized to achieve a hydrogel with a maximum swelling capacity. The maximum water absorbency of the optimized final product was 1238 g/g, while poly-2-acrylamido-2-methylpropanesulfonic acid-g-kappa-carrageenan and polyacrylic acid-g-kappa-carrageenan hydrogels swelled to a range of 135-800 g/g. The swelling of superabsorbent hydrogels was measured in various solutions with pH values ranging from 1 to 13. In addition, the pH reversibility, on-off switching behavior and swelling kinetics in distilled water were preliminarily investigated. (c) 2007 Elsevier Ltd. All rights reserved.