Determination of average molecular weight between cross-links ((M) over bar(c)) of itaconic acid-containing poly(vinyl pyrrolidone) and maleic acid-containing poly(acrylamide) hydrogels sensitive to pH changes of the swelling medium was investigated. Poly-electrolyte hydrogels with varying compositions were prepared in the form of rods from ternary mixtures of N-vinyl 2-pyrroridone/itaconic acid/water and acrylamide/ maleic acid/water. Gelation was achieved by irradiating the system with gamma rays at ambient temperature. The equation modified by the authors recently for the determination of (M) over bar(c) is observed to describe very well the swelling behaviour of charged polymeric networks. In addition to the evaluation of (M) over bar(c) from swelling data, the same equation also provides the simultaneous measurement of polymer-solvent interaction parameter of the systems investigated.