The swelling of KU-2 cation exchanger and AV-17 anion exchanger in the hydrogen, chloride, nitrate, sulphate and perchlorate forms were measured in mixed solvents. In mixed aqueous-organic solvents, preferential swelling by water was found. For the perchlorate form of the resin, the selective uptake of organic solvent was observed at N MeOH < 0.2 and at N Me 2O < 0.5. The addition of mineral acids decreased the selective uptake of water by resins, especially in the case of resins in the sulphate form. This can be explained by the capability of the anion exchanger to take up mineral acids. The preferential sorption of water strongly influences the sorption of Ni(II), Co(II) and Cu(II) ions.