Prediction of radionuclides migration through a geosphere is of fundamental interest in order to evaluate the safety of an underground radwastes repository. We have studied uranyl and europium(III) ion sorption mechanisms onto three phosphate compounds: ZrP2O7; Zr2O(PO4)(2), and Th-4(PO4)(4)P2O7. A spectroscopic investigation which has been previously reported has allowed one to determine all species involved in the sorption processes under study. This paper presents the modeling of experimental retention data using FITEQLv3.2 code (constant capacitance model). All sorption isotherms were successfully simulated with respect to the structural constraints. We have shown that uranyl sorption onto the solids under study is less influenced by electrostatic interactions between aqueous species and charges. on the surface than is that for europium(III) ion. Because; many experimental constraints, obtained from independent spectroscopic techniques, have been taken into account for the fitting procedure, the sorption constant values can be determined accurately.