Poly(N-vinyl 2-pyrrolidone/acrylic acid) IP(VP/AA)I polyelectrolyte networks were prepared from the ternary mixtures N-vinyl 2-pyrrolidone/acrylic acid/water by using gamma rays at ambient temperature. Equilibrium swelling behavior of these hydrogels was studied using an equation, based on the James-Guth phantom network theory and the approaches of Peppas et al., which was modified by the authors for determination of the (M) over bar (c) and chi parameters. The (M) over bar (c) and chi parameters were related to the equilibrium swelling behavior of the charged copolymeric network, pH, temperature, and ionic strength of the surrounding medium. The degree of swelling of these hydrogels was decreased with these variables, as expected. Oscillatory swelling behavior was also observed and investigated in response to changes in pH of the solution. (C) 2000 John Wiley & Sons, Inc.