This paper is concerned with solvent extraction studies that are used to study complex formation in the aqueous solution as well as in the organic solvent. In this hquid distribution method low metal concentrations can be used and the concentrations can generally be determined radiometrically. With proper choice of conditions, polynuclear reactions (e.g., formation of polynuclear hydroxo complexes) can be avoided, radioelements can be investigated, and the amount of complexing agent bound by the central ion can be neglected. Equilibria that have been investigated include the following types: (1) complexing in the aqueous phase with inorganic (e.g., OH−, Cl−, ) and organic ligands (e.g., CH3COO−, ), where the solvent extraction system is used to measure the concentration of the free metal ion; (2) complexing in both phases with extracting agents (e.g., acetylacetone, oxine); (3) residual coordination in neutral metal chelates or salts (e.g., UO2(NO3)2, UO2(TTA)2, UO2(R2PO4)2); (4) extraction of mixed chelate complexes. Mention is also made of systems that are not categorized by the above types.