Thirty-nine states in the US have fish consumption advisories due to PCB contamination, including six states that border the Ohio River. Several biogeochemical processes, acting in concert or individually, can attenuate PCBs in sediments; most processes, however, are dependent on environmental factors that regulate the growth and activities of specific microbial groups or assemblages. This study investigated the role of temperature, organic, and inorganic amendments on PCB removal and anaerobic microbial community composition in ¹⁴C-2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153)-spiked Ohio River sediments. Phylogenetic analysis of bacterial 16S rDNA clone libraries recovered from freshly collected sediments indicated a predominance of aerobic and facultative anaerobic bacteria and a lack of bacteria with PCB dechlorination ability, which indicated that native sediments were probably not sufficiently chemically-reduced for extensive reductive dechlorination of PCBs. However, PCB-153 was extensively meta-dechlorinated to 2,2',4,4'-tetrachlorobiphenyl (PCB-47) by native bacteria populations after they were exposed to extremely reducing (methanogenic) conditions. PCB-153 removal was greatest in sediments amended with peptone and volatile fatty acids as electron donors, but addition of ferrous sulfate as electron acceptor in combination with electron donors inhibited reductive dechlorination. Treatments showing extensive dechlorination were accompanied by increased occurrence of species in the Dehalococcoides, Desulfitobacterium, and Clostridium genera, which are associated with reductive dechlorination of PCBs and other chloroaromatics. It is proposed that bacterial community composition determined by phylogenetic analysis of bacterial 16S rRNA could be a useful tool for assessing certain biogeochemical conditions (e.g. temperature, redox, and electron availability) and so the likelihood of many processes in the environment, including reductive dechlorination of PCBs.