Previous work by Haas and DiChristina (ES&T 2002, 36) demonstrates that Fe(III) reduction by the facultative Fe(III)-reducing bacterium S. putrefaciens is mediated by competitive speciation among dissolved organic ligands and functional groups on the cell surface. Haas and DiChristina (2002) showed that rates of Fe(III) reduction by S. putrefaciens correlate with the thermodynamic stability constants of the Fe(III)-organic ligand complexes. The aforementioned work served as a comparative study for U(VI) reduction by S. putrefaciens. S. putrefaciens can also use U(VI) as a terminal electron acceptor, coupling U(VI) reduction to growth. In this study, S. putrefaciens was incubated in experimental media containing U(VI) in the form of aqueous complexes with a variety of organic ligands that differ significantly in structure and stability with respect to U(VI) chelation. The organic ligands used were maleate, malonate, citrate, oxalate, tiron, EDTA, salicylate, 5-sulfosalicylate, 5-chlorosalicylate, and 5-nitrosalicylate. Rates of U(VI) reduction by S. putrefaciens vary strongly as a function of U(VI) aqueous speciation. When present as aqueous complexes with citrate, tiron, and EDTA, U(VI) is not reduced by S. putrefaciens within the duration of our experiments (~100 hours). In contrast, the presence of salicylate, maleate and malonate ligands does not appear to significantly inhibit microbial U(VI) reduction. In contrast with Fe(III) reduction by this bacterium, inhibition of U(VI) reduction does not appear to vary as a simple function of thermodynamic stability constants of the appropriate 1:1 metal organic complex. These data will be presented along with discussion and interpretation of results.