Hydrogels of N-vinylimidazole (VI) and sodium styrenesulfonate (SSS) were synthesized in aqueous solution by radical crosslinking copolymerization with N,N-methylene-bis(acrylamide) as crosslinker. Swelling in several saline solutions was measured for hydrogel samples synthesized with different comonomer concentrations (C-T = 10, 25, or 40%) and with SSS mole fractions covering a broad range (f(SSS) = 0-0.7), while the crosslinker ratio was 2 wt % in all cases. The degree of swelling in aqueous solution with a specific ionic strength (mu), plotted versus the SSS composition of the feed, shows a minimum for any set of samples synthesized with a fixed C-T. The dependence of swelling on p shows both polyelectrolyte (f(SSS) beyond the minimum) and antipolyelectrolyte behaviors (in the low f(SSS) limit). It was found that the nonGaussian factor of the crosslinking density and the polymer-solvent interaction parameter increase with f(SSS) for any C-T. Moreover, in the low fSSS limit, the osmotic swelling pressure is governed not only by the ionic contribution, but also by the polymer-solvent mixing and, the concentration of mobile counterions inside the gel is not proportional to the net fixed charge but to the addition of cationic and anionic side groups, what discards the formation of ionic pairs. The antipolyelectrolyte effect is interpreted as due to the increasing protonation of VI as mu goes up. (c) 2007 Wiley Periodicals, Inc.