Semi-interpenetrating polymer network (IPN) hydrogels, with acrylic acid (AA) and poly(diallydimethylammonium chloride) (PDMDAAC), were constructed by a sequential IPN method. The characterizations of the lPN hydrogels were investigated by FTIR, DTA, and swelling tests under various conditions. The prepared semi-IPN hydrogels exhibited relatively high swelling capacity, in the range of 477-630 g/g at 25 degrees C. The results show that the swelling capacity of AA/PDMDAAC semi-IPN hydrogels was pH and temperature dependent. Swelling behaviors were also studied in the different salt solutions. Swelling kinetic parameters are given. (C) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 345-350, 2007