Prussian Blue, FeIII4[FeII(CN)6]3, and structurally related transition metal compounds are used as cesium ion exchangers in decontamination procedures of liquid radioactive waste. The used ion exchangers are conditioned as a cementitious waste form for interim storage and finally will become part of the radioactive waste in geological repositories. The problem discussed here is the long-term behaviour of the ion exchangers FeIII4[FeII(CN)6]3 and Ni2[Fe(CN)6] in planned geological repositories. The worst-case scenario is the instantaneous and complete dissolution and decomposition of the ion exchangers in the cementitious environment and the release of free cyanide. All radionuclides belonging to the class of “hard acceptors”, e. g. Cs, Sr, Ra, U, Np, Pu, Am and Cm, are not affected by cyanide complexation under these conditions. Radionuclides belonging to the class of “soft acceptors” and “borderline” metals, e.g. Co, Ni, Tc, Cd, Pb, Pd and Ag, are affected to various degrees by cyanide complexation. Strong complexation and extensive cyanide leaching is found for Ni, Co, Pd, Ag.