This study explored the geochemistry and microbial diversity of a Superfund site containing trichloroethene (TCE) and an unusual co-pollutant, tetrakis(2-ethylbutoxy)silane. Geochemical analysis of contaminated groundwater indicated subsurface anaerobiosis, reductive dechlorination of TCE to predominantly cis-1,2-dichloroethene, and (transient) accumulation of 2-ethylbutanol and 2-ethylbutyrate as a result of tetrakis(2-ethylbutoxy)silane breakdown. Comparative analysis of 106 16S rDNA and 61 16S-23S rDNA intergenic spacer region sequences - obtained from pristine and contaminated groundwater via DNA extraction, PCR amplification, cloning and sequencing - revealed that the contaminated groundwater featured (i) a distinct microbial community, (ii) reduced species diversity, (iii) various anaerobes, and (iv) bacteria closely related to the TCE-dechlorinating, dichloroethene-accumulating genus Dehalobacter, whereas (v) the TCE-dechlorinating, ethene-producing species Dehalococcoides ethenogenes was not detectable. Thus, geochemical and molecular biological results were in excellent agreement in this first ecological field study linking in situ reductive dechlorination of TCE to metabolism of tetraalkoxysilanes.