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A density functional theory (DFT) analysis has been conducted for the gas-phase hydrodechlorination (HDC) of dichloromethane (DCM) with palladium catalyst to achieve a better knowledge of the reaction mechanism involved in the HDC process, which constitutes an emerging technology for the treatment of organochlorinated contaminants. The computational study included the effect of size, oxidation state, and spin configuration of Pd cluster on the adsorption of H2 and DCM reactants on the catalyst surface. Calculations described the activation of H2 by Pd clusters through a dissociative adsorption with low enthalpy values. In addition, partially and fully dissociated DCM intermediates on Pd surface were predicted by DFT calculations. Remarkably, the dissociative adsorption of DCM on Pd active sites occurs via the scission of C–Cl bonds, promoted by the formation of C–Pd linkages, implying high adsorption enthalpy. The computational results showed that DCM can be also molecularly adsorbed on both zerovalent and electrodeficient Pd species. However, the nondissociative adsorption of DCM over electrodeficient Pd cluster is remarkably favored in energy, with adsorption enthalpies (∼−50 kcal/mol) corresponding to chemisorption. Current theoretical evidence explained the deactivation of Pd/AC catalyst as a consequence of the selective poisoning of electrodeficient Pd active centers by chlorinated hydrocarbons, in good agreement with our previous experimental findings.