We have determined the structure of uranyl, UO22+, and Th4+ complexes formed in aqueous solution with 4,5-dihydroxy-3,5-benzenedisulfonate (Tiron) as function of PH and concentration. At equimolar concentrations of 0.05 M UO22+ and Tiron, the predominant species was found to be aqueous uranyl at pH = 2.0. At pH = 6.0, the formation of a 3:3 UO22+:Tiron trimer (proposed in earlier studies) was observed. In this structure, bidentate catecholate complexation to Tiron as well as oxygen bridging between uranyl units is detected. Th4+ structural changes were observed both as a function of pH and Th:L (L = Tiron) ratio. At Th:L = 1:1 and pH = 1.4, a monomeric complex is observed with each Th center complexing monodentate to similar to2 sulfonate functional groups. At pH 4.0 similar sulfonate ligation is observed along with oligomer formation. At pH 6.0 thorium hydrolysis products are detected, with little evidence for inner-sphere Tiron coordination. When the Th:L is changed to 1:2 at pH = 6.0, a stable oligomeric complex is formed that dominates the speciation for Th:L ratios up to 1.5. This complex is characterized by bidentate catechol and monodentate sulfonate ligation to Tiron along with oxygen bridging between Th4+ atoms and is consistent with the formation of the 2:3 Th:L polymeric species proposed from earlier work. At a Th:L ratio of 1:10, Th4+ complexation is dominated by bidentate catechol ligation and the formation of a monomeric Th(Tiron), species, where x 1 2.