Palladium-decorated gold nanoparticles (Pd/Au NPs) have recently been shown to be highly efficient for trichloroethene hydrodechlorination, as a new approach in the treatment of groundwater contaminated with chlorinated solvents. Problematically, natural groundwater can contain chloride and sulfide ions, which are known poisons in Pd-based catalysis. In this study, the effects of chloride and sulfide on the trichloroethene hydrodechlorination catalytic activity were examined for non-supported Pd/Au NPs and Pd NPs, and alumina-supported Pd (Pd/Al2O3). Over the concentration range of 0-0.02 M NaCl, the catalytic activity of Pd/Au NPs was unaffected, while the activities of both the Pd NPs and Pd/Al2O3 Catalysts dropped by similar to 70%. Pd/Au NPs were found to be highly resistant to sulfide poisoning, deactivating completely at a ratio of sulfide to surface Pd atom (S:Pd-surf) of at least 1, compared to Pd NPs deactivating completely at a ratio of 0.5. Pd/Al2O3 retained activity at a ratio of 0.5, pointing to a beneficial role of the Al2O3 support. Sulfide poisoning of Pd/Au NPs with different Pd surface coverages provided a way to assess the nature of active sites. The gold component was found to enhance both Pd catalytic activity and poisoning resistance for room-temperature, water-phase trichloroethene hydrodechlorination. (C) 2009 Elsevier Inc. All rights reserved.