A series of hydrogels were prepared from acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) monomers with 0-80 mol % AMPS and using N,N'-methylenebis(acrylamide) as the crosslinker. The swelling capacities of hydrogels were measured in water and in aqueous NaCl solutions. The volume swelling ratio q(v) of hydrogels in water increases sharply when the mole fraction f(c) of AMPS increases from 0 to 0.06. At higher values off, from 0.06 up to 0.18, no change in the swelling capacities of hydrogels was observed; in this range of f(c), q(v) becomes nearly constant at 750. However, as f(c) further increases, q(v) starts to increase again monotonically over the entire range of f(c). At a fixed value of f(c), the swelling ratio of hydrogels decreases with increasing salt concentration in the external solution. The results of the swelling measurements in aqueous salt solutions were compared with the predictions of the Flory-Rehner theory of swelling equilibrium. It was shown that the theory correctly predicts the swelling behavior of hydrogels up to 80 mol % charge densities. The method of estimation of the network parameters was found to be unimportant in the prediction of the experimental swelling data. The network parameters used in the simulation only correct the deficiency of the swelling theory. (C) 1998 John Wiley & Sons, Inc.