The speciation of hexavalent uranium at the solid-solution interface was investigated. To experimentally identify the sorption equilibria, we characterized the structure of the surface complex formed by binding between uranyl ions and surface groups of solid matrixes. To understand the sorption mechanisms at a molecular level, we performed optical and X-ray photoelectron spectroscopies and X-ray absorption spectroscopy on uranyl ion loaded phosphate solids (LaPO4 and La(PO3)(3)). Two lanthanum phosphates were synthesized. The samples were contacted with aqueous uranyl solutions of pH values ranging from 1.0 to 4.0. Whatever the conditions, uranium surface coverage was always lower than 30% of the monolayer as measured by the proton-induced X-ray emission technique. The U 4f X-ray photoelectron spectra and the lifetime values of uranyl ions sorbed on the lanthanum monophosphate compound clearly evidence that this solid exhibits two different types of sorption sites, as well as lanthanum polytrioxophosphate. The nature of the site which interacts with uranyl ions also depends on the pH value for both solids. Moreover, the interaction of the uranyl ions and the phosphate solids in a nitrate medium leads to two different sorbed species: free aquo UO22+ ions and UO2(NO3)(+) ions. The X-ray absorption spectroscopy performed on the sorbed samples gives evidence of the presence of an inner-sphere mononuclear polydentate surface complex.