The swelling of poly(acrylamide) (PAAm) gels and the osmotic pressure of linear PAAm in aqueous solutions were predominantly affected by anion type and increased according to the lyotropic series ranking of sodium halide anions: F- < (H2O) < Cl- < Br-< I-. The osmotic pressure of PAAm in all examined salt solutions followed the scaling theory, with an exponent of 2.3 +/- 0.1. In solutions of a sodium halide series, the value of the pre-exponential factor seemed to depend on salt concentration, anion radius, and the apparent "anionic-portion radius" of the water molecule. This radius, extracted from the literature data, marks a transition point of the anion radius effect. Larger anions increase the osmotic pressure of PAAm more significantly as their concentration increases and vice versa. The effects of the anions on the osmotic pressure of PAAm are related to their preferential interactions with the polymer. Iodide, which increased the osmotic pressure of PAAm with respect to its value in pure water, seemed to preferentially adsorb onto the polymer with a binding constant of K-b = 9.7 +/- 2.0 M-1 determined by isothermal titration microcalorimetry. However, fluoride, which decreased the osmotic pressure, was preferentially repulsed. The mechanisms of attraction and repulsion were attributed to ion-water-polymer interactions and the solvent quality of the hydrated ions. (C) 2003 Wiley Periodicals, Inc.