Trichloroethylene (TCE) is an environmental contaminant that is subject to reductive degradation in natural and engineered systems. Through comparison of the final product ratio of cis-dichloroethylene (cDCE) to trans-dichloroethylene, obtained using well-characterized outer-sphere reductants to that of cobalamin, it was found that an outer-sphere electron transfer mechanism was not the likely major pathway in the reduction of TCE by cobalamin. To further explore the role of these intermediates in the catalytic cycle, the reduction of cis-1,2-dichlorovinyl(pyridine)cobaloxime (1) was studied. The reduction of 1 was performed by again applying well-characterized outer-sphere electron-transfer agents and cobalamin in the presence of H atom and proton donor species to identify the presence of chlorovinyl radicals and anions. The results provide evidence for a common intermediate arising from the reduction of this organometallic intermediate, a dichlorovinyl anion, which is involved in the production of the two main products in the cobalamin-mediated degradation of TCE: the cDCE and chloroacetylene. The results provide a further understanding of the cobalamin reduction of TCE and an explanation for the observed product distribution in natural and engineered systems. This is an abstract of a paper presented at the 231th ACS National Meeting (Atlanta, GA 3/26-30/2006).