A description is given of the preparation and chemical properties of copper(II) hexacyanoferrate (II) and (III). The first is well known and already used for rapid determination of137Cs in a wide variety of matrices, but the survey reveals that copper(II) hexacyanoferrate(III) is preferable as long as adsorption capacity is concerned. Thermal stability, reactions in acid media and competitive adsorption of K+ and NH 4+ ions are discussed for both adsorbers.