Bacteria capable of enzymatic U(VI) reduction are likely to govern the biogeochemistry of uranium in many subsurface settings. Previous studies have shown that common facultatively-anaerobic bacteria can carry out enzymatic reduction of aqueous U(VI) to relatively insoluble U(IV) in the laboratory. However, relatively little is known about the potential for U(VI)-reducing bacteria (URB) to carry out this reaction using adsorbed U(VI) surface complexes on aquifer materials. Labile U(VI) in natural systems exists predominantly as adsorbed species. In this study U(VI) adsorbed onto kaolinite and montmorillonite was supplied to the URB Shewanella putrefaciens in a basal salts medium under strict anaerobic conditions. Sterile controls were maintained, along with a biotic control containing a strain of S. putrefaciens incapable of U(VI) reduction. Uranium speciation was monitored by periodic extractions of U from aliquots using 1) H2SO4, 2) bicarbonate, and 3) filtration of neat supernatants, followed by analysis of dissolved U in extractions by ICPMS. Results demonstrate that S. putrefaciens readily reduces U(VI) adsorbed onto kaolinite, but reduces U(VI) sorbed onto montmorillonite much less effectively. These results suggest that adsorption of U(VI) onto some sedimentary minerals may limit the effectiveness of bioremediation strategies exploiting URB.