Microbial community responses to ethanol, methanol, and methanol plus humics amendments in relationship to U(VI) bioreduction were studied in laboratory microcosm experiments using sediments and ground water from a uranium-contaminated site in Oak Ridge, TN. The type of carbon source added, the duration of incubation, and the sampling site influenced the bacterial community structure upon incubation. Analysis of 16S rRNA gene clone libraries indicated that (i) bacterial communities found in ethanol- and methanol-amended samples with U(VI) reduction were similar due to the presence of Deltaproteobacteria and Betaproteobacteria (members of the families Burkholderiaceae, Comamonadaceae, Oxalobacteraceae, and Rhodocyclaceae); (ii) methanol-amended samples without U(VI) reduction exhibited the lowest diversity and the bacterial community contained 69.2 to 92.8% of the family Methylophilaceae; and (iii) the addition of humics resulted in an increase of phylogenetic diversity of Betaproteobacteria (Rodoferax, Polaromonas, Janthinobacterium, Methylophilales, and unclassified) and Firmicutes (Desulfosporosinus and Clostridium).