High affinity of thallium(I) ions with respect to a copper hexacyanoferrate (CuHCF,) film was found. Interaction between the CuHCF film and thallium(I) ions was investigated using the electrochemical quartz crystal microbalance and cyclic voltammetric techniques. In 0.5 M KNO3 solution, at submillimolar concentrations of TI(I) ions, the CuHCF film reversibly transforms, during electrochemical experiments, from the potassium into the thallium form. For slightly higher (several millimoles) Tl(I) concentrations, the electrochemical and gravimetric responses prove that the CuHCF film behaves as the thallium form only. Its formal potential (E-f(0)), calculated from the dependence of the E-f(0) on [TI(I)], is for [TI(I)] = I M, 0.28 V more positive than that found for the potassium form. Experimental results obtained suggest that in both cases, potassium(I) and thallium(I) ions, exchange in interstitial positions takes place. Since the ionic radii and the hydration parameters of both ions are similar, we concluded that this high affinity of thallium(I) ions with respect to the CuHCF film, 250-100 times higher than that of potassium(I) ion. results from chemical interactions. In consequence, the solubility product of a thallium analogue of copper hexacyanoferrate is much Smaller than that of the potassium form. For different forms they are equal to (pK values) 37.8 (K2Cu3[Fe(CN)(6)](2)). 46.3 (Tl2Cu3[Fe(CN)(6)](2)), and 17.2 (Cu-3[Fe(CN)(6)](2)).