The feasibility of immobilizing radionuclides on mineral surfaces was examined in the absence and the presence of phosphate anions, using trivalent lanthanide ions (Eu3+, Gd3+, and Dy3+) as chemical analogues of trivalent actinide radionuclides. The amount of lanthanide ions (Ln(3+)) sorbed on boehmite (gamma-AlOOH) surfaces dramatically increased in the presence of phosphate below pH 5. The structure of the sorbed lanthanide was determined by X-ray absorption spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance spectroscopy. We proved Dy3+ forms precipitates on boehmite surfaces in the presence of phosphate, and Gd3+ both in the presence and the absence of phosphate. In the presence of phosphate, however, these rare-earth cations react to form ultrafine particles of LnPO(4) surface precipitates on boehmite surfaces.