Hydrogels with various ionic group contents were prepared from acrylamide and crotonic acid (CrA) monomers with 0-12.9 mol% CrA in aqueous solutions by radiation-induced polymerization and gelation with gamma rays from a Co-60 source. The volume swelling ratio of the poly(acrylamide/crotonic acid) hydrogels was investigated as a function of the pH and ionic strength of the swelling medium and the type of counterion in the swelling medium. The volume swelling ratio increased with an increase in pH and a decrease in the ionic strength. The volume swelling ratio of these hydrogels was evaluated with an equation, based on the Flory-Huggins thermodynamic theory, the James-Guth phantom network theory, and the Donnan theory of swelling of weakly charged ionic gels, that was modified here for the determination of the molecular weight between crosslinks (M-c) and the polymer-solvent interaction parameter (x). The modified equation described very well the swelling behavior of the charged polymeric network. The same equation also provided the simultaneous measurement of these parameters for the systems investigated. (C) 2003 Wiley Periodicals, Inc.