All three isomers of trichlorobenzene were reductively dechlorinated to monochlorobenzene via dichlorobenzenes in anaerobic sediment columns. The dechlorination was specific: 1,2,3- and 1,3,5-trichlorobenzene were solely transformed to 1,3-dichlorobenzene, while 1,4-dichlorobenzene was the only product of 1,2,4-trichlorobenzene transformation. Microorganisms were responsible for the observed transformations. Since monochlorobenzene and dichlorobenzene are mineralized by bacteria in the presence of oxygen, the process of reductive dechlorination may be an important initial step to obtain complete mineralization of otherwise recalcitrant trichlorobenzenes. This is especially true for the 1,3,5-isomer, which seems to resist biodegradation in oxic environments.