Deuterium isotope exchange was employed to investigate some of the reaction steps for the hydrodechlorination of CF3CFHCl and CH3CH2Cl on supported Pd catalysts. Hydrodechlorination rates for both compounds using D-2 or H-2 were identical. It is concluded that H or D species are not involved in the rate-determining step of the hydrodechlorination reaction. Experiments using a combined H-2 and D-2 feed for hydrodechlorination of CF3CFHCl showed that CF3CFH2 was preferentially formed over CF3CFHD. This experimental observation can be explained by isotope effects and is consistent with gas phase H-2 in equilibrium with adsorbed H and a reaction step consisting of the combination of surface adsorbed CF3CFH- species with adsorbed H. Deuterated product distributions of CF3CFHCl and CH3CH2Cl hydrodechlorination experiments with D-2 were analyzed to explore the reaction steps beyond the kinetically relevant steps. The main product of CF3CFHCl hydrodechlorination was CF3CFHD, showing that the C-Cl bond is preferentially broken. In the case of CH3CH2Cl, substantial C-H bond breaking accompanies the initial C-Cl scission. Deuterium exchange experiments with ethane on Pd/C and Pd/ZrO2 were also investigated and the results indicate that the CH3CH2Cl product distribution is not caused by re-adsorption and subsequent exchange reaction of ethane. (C) 2004 Elsevier B.V. All rights reserved.