Dechlorination patterns of three isomers of tetrachlorobenzenes, i.e. 1,2,3,4-, 1,2,3,5-, 1,2,4,5-TeCB, and associated microbial communities were studied in anaerobic microcosms derived from a contaminated harbor. The removal of doubly, singly and un-flanked chlorine atoms was noted in the 1,2,3,4- and 1,2,3,5-TeCB dechlorinating microcosms, whereas only singly flanked chlorine was removed with 1,2,4,5-TeCB as the respiratory electron acceptor. Di- and/or mono-chlorobenzene (MCB) were observed as the main end products in the reductive dechlorination of all three TeCB isomers. Interestingly, among various potential dechlorinating pathways of TeCB isomers, the thermodynamically more favorable reactions were selectively followed by the enriched microcosms. Bacterial community profiling was applied in combination with quantitative PCR (qPCR) of 16S rRNA genes to gain an insight into TeCB dechlorinators and their putative interactions with non-dechlorinating bacteria within the community. Overall, our results provide a better understanding of microbial guilds enriched and active in anaerobic TeCB dechlorinating microcosms derived from estuarine environments.