In this study, N-isopropylacrylamide-acrylic acid (NIPAAm-AAc) copolymer hydrogels were synthesized by radical polymerization. The equilibrium and dynamic water uptake of these copolymer hydrogels were investigated as a function of acrylic acid content of copolymer, pH, and temperature. The pH-dependent swelling ratio of the hydrogels was distinguishable between two pH-modulating methods. In addition, a modified thermodynamic simulation that is able to predict pH-dependent swelling of polyanionic gels was developed. In the simulation, the influence of ionic strength on dissociation of carboxyl groups was taken into consideration. Also a new relation between Debye length and volume change of gels was derived from the approximation used in derivation of ionic osmotic pressure. The predictions from this modified equation were reasonably good compared with experiments. (c) 2006 Wiley Periodicals, Inc.