Cu II 2 Fe II (CN) 6 • xH 2 O and Cu II 3 [Fe III (CN) 6 ] 2 • xH 2 O can be prepared with reproducible chemical compositions and structures after careful washing. They have cubic Fm3 m structures with iron vacancies. In Cu II 2 Fe II (CN) 6 , copper occupies two different sites: Cu1 in position 4b linked to Fe through the CN groups, and Cu2 not linked to the CN groups and partially occupying the interstitial 24e positions. The second type of site is not present in Cu II 3 [Fe III (CN) 6 ] 2 . Sorption kinetics and isotherms were determined for cesium on both hexacyanoferrates by batch experiments. On Cu II 3 [Fe III (CN) 6
] 2 , the maximum uptake is only 0.073 Cs/ Fe (at./at.). On Cu II 2 Fe II (CN) 6 , the uptake reaches 1.5 Cs/ Fe. The sorption kinetics include at least two steps: a t 1/2 variation until approximately 72 h and then a slow evolution studied up to 6 months. The sorption mechanism is complex. The main process seems to be diffusion of ion pairs, followed by a reorganization of the solid, resulting in one or more new solid phases. The presence of the Cu2 site seems to play a favorable role in the sorption. Owing to its good midterm stability and the first rapid step of exchange, Cu II 2 Fe II (CN) 6 • xH 2 O seems to be one of the most promising compounds for the recovery of cesium from nuclear liquid wastes.