Hydrogels containing triprotic acid moieties sensitive to pH and ionic strength changes of the swelling medium were prepared from the ternary systems N-vinyl 2-pyrrolidone/citric acid/water by irradiation with gamma rays at ambient temperature. Equilibrium swelling behavior of these hydrogels was studied using an equation, based on the phantom network theory of James-Guth and the approaches of Peppas et al., which was modified by the authors for determination of (M) over bar(c) and chi parameter. The (M) over bar(c) and chi parameters were related to equilibrium swelling behavior of the charged polymeric network, pH, temperature, and ionic strength of the surrounding medium. The degree of swelling of these hydrogels decreases with these variables in accordance with expectation. (C) 2000 John Wiley & Sons, Inc.