An apparent stall in the biodegradation of 1,2-cis-dichloroethene (cDCE) is often observed at many natural attenuation sites where cDCE accumulates and is not further degraded. The lack of further breakdown cDCE is often attributed to a lack of available hydrogen donor and/or absence of a suitable microbial community to further degrade the contaminant. Bradley et al. (1998) reported that addition of Mn(IV) could enhance microbial oxidation of cDCE under anaerobic conditions. However, the extent to which this process occurs in groundwater and whether it can be enhanced by manganese dioxide (MnO2) addition in aquifers with persistent cDCE is unknown. This study, funded by the Environmental Security Technology Certification Program (ESTCP Project No. ER-0625), has examined the effect of MnO2 and other amendments in promoting biological oxidation of cDCE under anaerobic, aerobic or cometabolic conditions. Efforts were also made to find and enrich for naturally-occurring microbial populations that could biodegrade cDCE using MnO2 as an electron acceptor.