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We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium͑VI͒ was supplied as sole terminal electron acceptor to Shewanella putrefaciens ͑strain 200R͒ in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U͑VI͒ and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U͑VI͒, resulting in removal of insoluble amorphous UO 2 from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U͑VI͒ reduction to U͑IV͒, to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U͑VI͒:ligand and U͑IV͒:ligand complexes. In the presence of strongly complexing ligands ͑e.g., NTA, Tiron, EDTA͒, UO 2 precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO 2 , which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U 4ϩ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U 4ϩ , which is a necessary step in UO 2 precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U͑VI͒-organic complexes may also influence overall UO 2 production rates, although the magnitude of this effect is unclear relative to the effects of U͑IV͒-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO 2 .