Researchers tested new reactive barrier technologies with material from an aquifer in Bitterfeld, Saxonia-Anhalt, Germany, which is affected mainly by chlorobenzene (CB). A reactor filled with original aquifer sediment was designed for the microbiological remediation of the ground water by indigenous bacterial communities. The study examined the degradation of CB under two conditions: anoxic conditions in the presence of nitrate and mixed electron acceptor conditions (oxygen+nitrate) using hydrogen peroxide as the oxygen-releasing compound. No definite degradation of CB was observed under anoxic conditions. Adding hydrogen peroxide (2.94 mM) and nitrate (2 mM) led to the disappearance of CB in the lower part of the reactor, accompanied by a strong increase of the number of cultivable aerobic CB-degrading bacteria in reactor water and sediment samples, which indicates that CB was degraded mainly by productive bacterial metabolism. In laboratory experiments with reactor water containing CB-degrading bacteria (mostly Pseudomonas and Rhodococcus) oxygen was rapidly released by hydrogen peroxide, whereas biotic-induced decomposition reactions of hydrogen peroxide were almost four times faster than abiotic-induced decomposition reactions. No clear chemical degradation of CB mediated by hydrogen peroxide was observed. CB was also completely degraded in the reactor after reducing the hydrogen peroxide concentration to 880 uM. The CB degradation completely collapsed after reducing the hydrogen peroxide concentration to 440 uM. When the hydrogen peroxide concentrations were increased again, the oxygen demand for CB degradation was higher than observed before, indicating a shift in the bacterial population. Throughout the experiment, nitrate was uniformly reduced in the reactor flow path.