A two-stage anaerobic-aerobic biofilm reactor successfully degraded a mixture of chlorinated organic compounds to water-soluble metabolic intermediates and carbon dioxide. Reductive dechlorination of hexachlorobenzene (HCB), tetrachloroethylene (PCE), and chloroform (CF) occurred on all tested primary carbon sources such as glucose, methanol, and acetate. However, the extent of dechlorination was maximum when the anaerobic biofilm column was fed acetate as a primary carbon source. HCB, PCE, and CF were dechlorinated to the levels of tri- and dichlorinated products (99, 80, and 32%, respectively) with acetate in the feed. This is important, since these less-chlorinated compounds can be metabolized by the aerobic biofilm. The effluent from the anaerobic biofilm column was fed directly into the aerobic column. After both columns, the total amount transformed into nonvolatile intermediates and carbon dioxide was 94, 96, and 83% for [14C]HCB, [14C]trichloroethylene, and [14C]CF, respectively. This research shows the potential application of this novel two-stage bioreactor system for treating groundwaters and industrial effluents composed of highly chlorinated aliphatic and aromatic hydrocarbons.