Vitamin B-12 catalyzes the reductive dechlorination of several ubiquitous pollutants including the conversion of trichloroethylene (TCE) to similar to95% cis-1,2-dichloroethylene (DCE) and small amounts of trans-DCE and 1,1-DCE. The origins of this unexpected selectivity were investigated using density functional and coupled-cluster theory. At all levels of theory considered, the initially formed trichloroethylene radical anion is an unstable species. Breakage of one of the three C-Cl bonds during the dissociative process gives the most stable ion complex when the two remaining chlorines occupy a cis geometry. Once formed, the cis-1,2-dichloroethen-1-yl radical is about 6 kJ/mol more stable than the corresponding trans radical and 21 kJ/moI more stable than the 1,1-dichloroethen-2-yl radical. The calculated relative energies can be rationalized by delocalization of the impaired electron over the nonbonding orbitals of the alpha-chlorine. The computed geometries of the radicals suggest significant interactions between the orbital occupied by the unpaired electron and the sigma* orbital of the beta C-Cl bond trans to the radical. The barrier for interconversion of the two 1,2-dichlorinated vinyl radicals lies between similar to30-40 kJ/mol depending on the level of theory. The reactivities of the three radicals with respect to hydrogen atom abstraction from methanol (C-H or O-H) as well as chlorine elimination were investigated, All three radicals show a strong preference for abstraction of the alpha-hydrogen atom of methanol (17-25 kJ/mol), with a significant positive reaction energy for chlorine elimination (6080 kJ/mol). These results are discussed further in relation to the experimentally observed product distribution.