Complexation between glutar-imidedioxime and uranyl ion in artificial seawater containing 600 ppm uranium at pH 10.3 was investigated by a spectroscopic method. The complexation rate and the complex concentration at equilibrium increased with decreasing concentration of total carbonate ions. In the range of the total carbonate concentration of 4 x 10(-3) to 5 x 10(-2) mol.L-1, the complexation proceeded by ligand exchange between a carbonate ion and a deprotonated imidedioxime. At lower total carbonate concentrations, the elimination of more than two carbonate ions from one uranyl ion was suggested.