The graft copolymer of acrylic acid on biopolymer chitin, with 45% grafting efficiency, was used to study the effect of the carboxylic group of the graft copolymer on the metal binding ability of calcium ions in aqueous solution as a function of pH, contact time and metal concentration. The adsorption equilibrium data correlate well with the Langmuir isotherm equation. The maximum adsorption capacities of graft copolymer and chitin were found to be 0.500 +/- 0.013 and 0.195 +/- 0.005 mmol Ca2+ g(-1), respectively, suggesting the effective participation of the carboxylic group in Ca2+ adsorption. The poly(acrylic acid)-grafted chitin is promising as a polymeric matrix to be used in the preparation of new glass ionomer cements.