The synthesis of cyanoferrates of quadrivalent metals and the sorption of caesium ions by these materials have been studied. It has been found that the compositions of the compounds and their properties are strongly influenced by the degree of hydrolysis of the metal. These is no sorption of caesium by Zr4+ and Th4+ cyanoferrates, while the compounds of oxygencontaining bivalent complex cations exhibit behaviour similar to that of the other bivalent metals. The sorption of caesium by (TiO)2R is govermed by an ion-exchange mechanism; with (ZrO)2R changes occur in the crystal lattice, and (ThO)2R exhibits a low affinity towards caesium.