Reductive dehalogenation of tetra- and tri-chloroethene to cis- and trans-1,2-dichloroethene in microcoms simulating ground water environment has previously been demonstrated. In the study, anoxic microcosms containing organic sediment and water were spiked to contain 5 mg/L of one of the following compounds: 1,1-dichloroethene (1,1-DCE), cis-1,2-dichloroethene (CIS), or trans-1,2-dichloroethene (TRANS). After incubation in the dark at 25 deg C for up to six months, contents were analyzed by gas chromatography, and verified by GC/MS in an attempt to identify sequential steps in the transformation process. Vinyl chloride (VC) was produced after 1 to 2 weeks of incubation in all spiked microcosms, but none was observed in sterile and unspiked controls. Chloroethane (CE) was produced only in microcosms spiked with CIS, indicating isomer specificity and the occurrence of mechanisms other than reductive dechlorination. Kinetics parameters associated with the microbial dehalogenation of 1,1-DCE, CIS, and TRANS were calculated.