A five-year site investigation was conducted at a former chemical plant in Nanjing, China. The main contaminants were 1,2,4-trichlorobenzene (TCB) reaching concentrations up to 7300 μg/L, dichlorobenzene (DCB) isomers, monochlorobenzene (MCB), and benzene. Over time, these contaminants naturally attenuated to below regulatory levels under anaerobic conditions. To confirm the transformation processes and to explore the mechanisms, a corresponding laboratory microcosm study was completed demonstrating that 1,2,4-TCB was dechlorinated to 1,2-DCB, 1,3-DCB, and 1,4-DCB in approximately 2%/10%/88% molar proportions. The DCB isomers were dechlorinated via MCB to benzene, and, finally, benzene was degraded under prevailing sulfate-reducing conditions. Dechlorination could not be attributed to known dechlorinators Dehalobacter or Dehalococcoides, while anaerobic benzene degradation was mediated by microbes affiliated to a Deltaproteobacterium ORM2, previously associated with this activity. Unidentified organic compounds, possibly aromatic compounds related to past on-site production processes, were fueling the dechlorination reactions in situ. The microcosm study confirmed transformation processes inferred from field data and provided needed assurance for natural attenuation. Activity-based microcosm studies are often omitted from site characterization in favor of rapid and less expensive molecular surveys. However, the value of microcosm studies for confirming transformation processes, establishing electron balances, assessing cocontaminant inhibition, and validating appropriate monitoring tools is clear. At complex sites impacted by multiple compounds with poorly characterized transformation mechanisms, activity assays provide valuable data to incorporate into the conceptual site model to most effectively inform remediation alternatives.