The demand for effective and inexpensive treatment for decontamination of waters from radionuclides, such as 137 Cs, is presently high. In this context a selective adsorbent material for Cs (SORBMATECH 1 202) was designed, consisting of potassium/copper ferrocyanide nanoparticles deposited in the mesoporosity of silica grains. Several batch experiments were carried out in order to obtain kinetic and thermodynamic data concerning the ion exchange. Isotherms and measurements of distribution coefficients (K d,Cs ) in waters of increasing salinity (pure water, Ca-bicarbonate fresh water and seawater) were conducted in a radioactive environment. This study shows a fast ion-exchange kinetics (<5 min), due to the open silica porosity combined to cyanoferrate nanoparticles, as well as high selectivity with K d,Cs in fresh and sea waters (10 6 and 2.0 Â 10 5 mL/g, respectively). These experiments also demonstrated the competitive effects of the major cations present in natural waters (K + , Na + , Mg 2+ and Ca 2+ ) and led to the determination of the Vanselow’s selectivity coefficients. Integrating this dataset into the CHESS geochemical speciation code allowed the correct modeling of K d,Cs values in these different water types over a wide range of Cs concentrations. The dataset can be extrapolated to the modeling of other K- ferrocyanides and effluent compositions.