Arsenic (As) and chlorinated solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) are widespread groundwater contaminants due to natural processes and/or anthropogenic activities. Common bioremediation strategies involved in the removal of chlorinated solvents often establish suitable redox conditions that facilitate As dissolution and mobilization, but the effects of arsenic speciation on microbial dechlorination activity remain largely unexplored. In this study, we tested the activity of several dechlorinating pure cultures at the presence of varying concentrations of Arsenite (As (III)) and Arsenate (As (V)). As (III) exhibited more toxic effects than As (V) on the tested dechlorinating bacteria, and the presence of either As species significantly inhibited microbial dechlorination activity and resulted in slower rates and undesirable dechlorination end points. As (V) (5 μM and 50 μM) was quickly reduced to As(III) within 24 hours in a PCE-dechlorinating microcosm inoculated with aquifer solids. PCE-to cis-1,2-dichloroethene (cDCE) dechlorination was not inhibited, but the further detoxification of cDCE to ethene was slow and incomplete. In the microcosm amended with an initial 50 μM As (V), PCE dechlorination was completely stopped at the stage of cDCE and no vinyl chloride or ethene was detected. 16S rRNA high-throughput sequencing results revealed that the abundance of Dehalococcoides mccartyi was decreased by 19.3% and 71.1% when exposed to 5 μM and 50 μM As(V), respectively, which is consistent with the observed loss in cDCE dechlorination activity. These results suggest that As con-contamination has a negative impact on the bioremediation of chlorinated solvents and the arsenic biogeochemical processes need to be taken into consideration during site restoration practice.