Relationships between the formation mechanism and the swelling behavior of acrylamide (AAm)/2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS)-based hydrogels were studied. The hydrogels were prepared by free-radical crosslinking copolymerization of AAm and AMPS at 40 degrees C in the presence of N,N'-methylenebis(acrylamide) (BAAm) as the crosslinker. Both the crosslinker ratio (mole ratio of crosslinker to monomer) and the initial monomer concentration were fixed at 1/82 and 0.700 M, respectively, while the AMPS content in the monomer mixture was varied from 0 to 100 mol%. It was found that the copolymer composition is equal to the monomer feed composition, indicating that the monomer units distribute randomly along the network chains of the hydrogels. The monomer conversion versus time histories as well as the growth rate of the gel during the polymerization were found to be independent of the amount of AMPS in the initial monomer mixture. It was shown that the reaction system separates into two phases at the gel point and the gel grows in a heterogeneous system. The equilibrium degree of swelling of the final hydrogels increases with increasing AMPS content until a plateau is reached at about 10 mol% AMPS. Between 10 and 30 mol% AMPS, the equilibrium gel swelling in water as well as in aqueous NaCl solutions was independent on the ionic group content of the hydrogels. Further increase in the AMPS content beyond this value increased the gel swelling continuously up to 100 mol%. The polyelectrolyte theories based on the counterion condensation cannot explain the observed swelling behavior of AAm/AMPS hydrogels. The swelling curves of the hydrogels in water and in aqueous NaCl solutions were successfully reproduced with the Flory-Rehner theory of swelling equilibrium including the ideal Donnan equilibria, where the effective charge density was taken as an adjustable parameter. Scaling rules were derived for the ionic group content and the effective excluded volume of the hydrogels. (C) 2000 Elsevier Science Ltd. All rights reserved.