Perchloroethene (PCE) is a common groundwater contaminant, due to its common use as a dry-cleaning solvent. Current treatment methods are limited in their ability to remove PCE from contaminated sites in an efficient and cost effective manner. Palladium-on-gold nanoparticles (Pd-on-Au NPs) have been shown to be highly catalytically active in the hydrodechlorination (HDC) of trichloroethene (TCE) and other chlorinated compounds. However, the catalytic chemistry of such nanoparticles for PCE HDC in water has not been systematically addressed in the literature. In this paper, we assess the catalytic properties of similar to 4 nm Pd-on-Au NPs, similar to 4 nm Pd NPs, and Pd/Al2O3 for water-phase PCE HDC under ambient conditions. The Pd-on-Au NPs exhibited volcano-shape activity as a function of Pd surface coverage (sc). Maximum activity was at 80 +/- 0.8 sc% (pseudo-first order rate constant of similar to 5000 L/g(Pd)/min), which was similar to 20x and similar to 80x higher than that for Pd NPs and Pd/Al2O3 at room temperature and pH 7. A complete mechanistic model of PCE HDC that coupled gas-liquid mass transfer with the surface reactions was developed and found to be consistent with the observed concentration-time profiles for the 3 catalyst types. The formation and subsequent reaction of daughter products (TCE, dichloroethene isomers, vinyl chloride, and ethene) followed the stepwise dechlorination of the PCE chlorine groups. The final reaction products were ethane and minor amounts of n-butane/butenes. This study establishes the enhanced degradation chemistry of PCE using model Pd-on-Au catalysts and suggests the volcano-shape structure-activity dependence can be generalized from PCE and TCE to other organohalides. (C) 2013 Elsevier B.V. All rights reserved.