Throughout 6 yr of operation, the pilot Rheine ZVI CRB has consistently shown constant degradation rates of perchloroethylene (PCE) at rates > 98%, and it is assumed that combined abiotic and biotic effects inside the barrier significantly contribute to this successful performance. Microbial activities at the Rheine PRB for remediation practice and long-term performance of PRB were studied. Microbial activity led to significant consumption of molecular hydrogen, which was produced abiotically through the anaerobic corrosion of zero valent iron. ReSponge™ seemed to be superior to other ZVI types regarding long term degradation efficiency, likely due to its specific characteristics (larger granules, but amorphous with a high internal surface, special composition). Therefore, it provided high dechlorination potential towards the chlorinated contaminants even after 7 yr of operation of the Rheine barrier. Besides the consumption of H₂, homo-acetogenic and methanogenic bacteria could help reduce excessive carbonate precipitation by converting dissolved CO₂ into acetate and methane, respectively. However, direct biological contribution by means of dechlorination could not be observed. This is an abstract of a paper presented at the proceedings of the 8th International In Situ and On-Site Bioremediation Symposium (Baltimore, MD 6/6-9/2005).