Studies of the complexation of U(vi) with amidoxime-related ligands help in the development of efficient sorbents for the extraction of uranium from seawater. In the present study, the stability constants of the U(vi) complexes with two phthalimidedioxime ligands were determined by potentiometry and spectrophotometry, and compared with glutarimidedioxime previously studied. Density functional theory calculations were performed to identify the most probable protonation sites of the ligand and to help interpret the trend in the binding strength of the ligands. The phthalimidedioxime complexes were found to be 2-3 orders of magnitude weaker than the corresponding glutarimidedioxime complexes, which was attributed to the difference between the ligands in the electronic and structural properties. The incorporation of the aromatic ring into phthalimidedioxime reduces the electron density on the donor atoms of the ligand and makes the imidedioxime moiety less complementary for binding UO2(2+)via its equatorial plane. Though weaker than glutarimidedioxime, phthalimidedioxime still forms fairly strong U(vi) complexes and can still effectively compete with carbonate for the complexation of U(vi) at seawater pH and carbonate concentration. Due to its higher chemical stability in acidic solutions than that of glutarimidedioxime, phthalimidedioxime is a valuable ligand that could have potential use in the extraction of U(vi) from seawater.