A microcosm test was designed to study the bioreduction rates of uranium(VI) and microbial community changes in a contaminated sediment under varying electron donor and aqueous geochemical conditions. The microcosms contained a highly contaminated sediment (with ~2.8 g U/kg) and were amended with two concentration levels of bicarbonate (1 and 40 mM) and two levels of sulfate (1.1 and 2.6 mM). Microcosms were incubated at ambient temperature (22-24°C) using ethanol and acetate as electron donor sources. Results indicate that, after 47 days of incubation, biological reduction of U(VI) and sulfate occurred in all microcosms. However, significantly higher bioreduction rates of U(VI) were observed with the amendment of ethanol than acetate in all microcosms, although the reduction rate decreased at the high sulfate concentration level. Acetate did not increase the rate of U(VI) reduction at the high bicarbonate concentration level. Microbial communities were dominated by species of Geothrix genus and Proteobacteria phylum, regardless of treatments of different electron donors or bicarbonate and sulfate concentrations. Species in Geobacteraceae family were significantly enriched at the low bicarbonate concentration level with ethanol and acetate treatments and likely responsible for the uranium bioreduction because they have been known to reduce U(VI). However, they were not enriched at the high bicarbonate concentration level, suggesting that Geothrix and the unclassified betaproteobacteria played important roles in U(VI) bioreduction under such conditions. This study demonstrates that different electron donors and geochemical conditions significantly impact the dynamics of microbial community and thus the bioreduction rates of U(VI), and these factors should be of consideration in the development of strategies for remediating U(VI)-contaminated sites.