In order to investigate the design concept of highly ion selective extractants for f-element series, some stability constants of trivalent f-element/dicarboxylate complexes were determined by solvent extraction method in 0.1 M NaClO4 at 25 degrees C. The negatively charged oxygen donors in terminal carboxyl groups can mainly contribute to stabilize the metal complex. On the other hand, secondary amine groups in the dicarboxylic acid enhance stabilization of a series of metal complexes. Especially, the soft donor atom such as nitrogen, which can coordinate covalently to 5f-elements, shows the higher selectivity of 5f elements front 4f ones than the harder oxygen atom. The role of nitrogen donor atoms in the carboxylic acid is also discussed with the well-known multidentate ligand EDTA. (c) 2005 Elsevier B.V. All rights reserved.