The predictions of the Flory-Rehner theory of swelling equilibrium including the ideal Donnan equilibria were compared with the experimental swelling data obtained from poly(acrylamide-co-sodium acrylate) hydrogels swollen in water and in aqueous salt (NaCl) solutions. For this comparison, the fraction of counterions which are effective in the gel swelling was taken into account. The ionic hydrogels were prepared from acrylamide and sodium acrylate (NaAc) monomers with 0 to 5 mol% NaAc and using N,N'-methylenebis(acrylamide) as the crosslinker: As expected. at a fixed crosslinker ratio, the volume swelling ratio of hydrogels in water increases sharply when the mole fi action of NaAc increases or the NaCl concentration in the external solution decreases. Taking into account the wasted counterions within the hydrogels, the theory correctly predicts the swelling behavior of hydrogels in water and in aqueous salt solutions, Not predicted by the theory is the increased swelling ratio of hydrogels with less than 4 mol% NaAc content with increasing salt concentration from 10(-1) to 10(0) M. (C) 1999 Elsevier Science Ltd. All rights reserved.