The swelling of dextran gels (Sephadex) in salt solutions with a water activity of 0.937, compared with the swelling in pure water, exhibited anion specificity as evidenced by an increased swelling ratio in the following order: Na2SO4 < H2O < NaCl < NaSCN. The swelling ratio showed a good linear correlation with the osmotic pressure of dextran (500 kD) in these solutions. The salt-concentration difference (imbalance) between the polymer-solution side of the membrane and the polymer-free permeate side during the osmotic-pressure measurements positively correlated with the effect of the salt on the polymer osmotic pressure. These phenomena conform to Hofmeister-type (or lyotropic) behavior. The diminishing augmentation of dextran osmotic pressure and the change in the salt-concentration imbalance with rising NaSCN concentration imply a positive preferential interaction and adsorption of the salt onto the polymer.