The stability constants of the complexes of Fe(III), Cu(II), Pb(II), and Ni(II) with glutarimidedioxime, a ligand that has significant implications in the sequestration of uranium from seawater with amidoxime-based sorbents, were determined by potentiometry. The enthalpies of complexation for the Fe(III)/glutarimidedioxime complexes were determined by microcalorimetry. Compared with the data for the complexation of U(VI) with glutarimidedioxime in the literature, Fe(III) forms much stronger complexes with glutarimidedioxime and is expected to affect the efficiency of uranium sequestration from seawater by competing for the sorption sites of the amidoxime-based sorbents. The structures of two Fe(III)/glutarimidedioxime complexes were identified by single-crystal X-ray diffractometry, and helped to interpret the stronger complexation of Fe(III) with glutarimidedioxime than that of U(VI).