[en] The processes at the solid/solution interfaces lead generally to the retention/migration of radionuclides. In order to quantitatively estimate these phenomena and obtain the corresponding thermodynamic constants, it is necessary to model the retention experimental data. Nevertheless, in order to simulate these data at best, in parallel to the macroscopic studies, it is indispensable to characterize these interactions at the molecular scale by a structural approach of the interface (carried out with different spectroscopic techniques). Indeed, by this approach, in using the sorbed cation as structural probe, it is possible to define experimentally the different components occurring in sorption equilibria. In order to illustrate this methodology, several systems (substrate/radionuclide) will be presented. (O.M.)