For active materials of the all-uranium redox flow battery for power storage, two tetraketone ligands, which possess two monomer acetylacetone moieties, were investigated in terms of the complexation with uranium. Detailed NMR measurements were conducted to reveal the keto–enol tautomerism of the tetraketones in CDCl 3 and titration measurements were carried out in water–dioxane (1:1 (v/v)) solutions to evaluate formation constants with metal ions at III–VI valences. Although the first acid dissociation constants for tetraketones are close to that for the acetylacetone, the formation constants of tetraketones at large coordination numbers are larger than those of acetylacetone. On the basis of these formation constants, the thermodynamic distributions of tetraketone complexes are evaluated in the solution to demonstrate that the change in the coordination number is not expected during the redox reactions contrary to the case of the acetylacetone.