A palladium-on-zeolite catalyst has been optimized for treating groundwater contaminated with halogenated hydrocarbon compounds (HHCs) by hydrodechlorination with dissolved hydrogen. Aqueous sulfite was used as the model poison and the dechlorination of 1,2-dichlorobenzene (1,2-DCB) was used as the model reaction to study the relationship between zeolite hydrophobicity, pore size, and resistance against deactivation. The optimal Pd support is a hydrophobic zeolite Y with a pore size of 0.74 nm tailored to exclude reactive ions from internal sites, to minimize deactivation and inhibition, and to maximize the transformation rate of HHCs. Common halogenated contaminants including chlorinated ethylenes and aromatics are removed within minutes. Catalyst efficiency under groundwater treatment conditions was demonstrated in a bench-scale column experiment. (C) 2000 Elsevier Science B.V. All rights reserved.