AbstractEquilibrium swelling and salt accumulation were investigated for highly swollen polymethacrylic acid gels in aqueous salt solutions of 0.8 × 10−2to 0.5MLiCl, NaCl, and KCl, at varying degrees of ionization of the polymer. The same equilibrium results were obtained with all three cations, thus showing that in the range investigated, specific interactions between the cations and the polyacid network may be disregarded. It was shown that the Donnan distribution, uncorrected for activity factors, cannot be applied to highly charged, highly swollen gels, even as a rough approximation. A general theory was developed which satisfactorily describes these equilibrium results in the entire ionization range of the gel in salt solutions up to 0.5M. This theory was based on Flory's equation for the contractile free energy of the network, and on the equation of Katchalskyet al. for the electrostatic free energy of polyelectrolyte solutions. It was found that the activity factor of the salt in the gel‐phase is chiefly determined by the electrostatic factor, and closely approximates the activity factor in polyelectrolyte solut