Solvent effects in the liquid phase (0.1 MPa; 303 K) hydrodechlorination (HDC) of 2,4-dichlorophenol have been established over Pd/Al2O3. In the absence of secondary reactions, catalyst deactivation, and transport limitations, a stepwise HDC yields 2-chlorophenol and phenol, where product selectivity, was insensitive to the nature of the solvent. In contrast, the initial HDC rates exhibited a marked dependence on the reaction medium and increased in the order: benzene < THF < n-hexane < cyclohexane < alcohols < water. Higher rates result from the concomitant effect of an increase in the dielectric constant (epsilon) and a decrease in the molar volume (<(nu)over bar>) of the solvent, where the major (ca. 80%) contribution is due to epsilon. We attribute this response to the increased solvent capacity to stabilize the arenium intermediate at higher/lower epsilon/(nu) over bar, an effect that extends to reaction in water + organic combinations. We provide, for the first time, a reliable quantification of solvent effects that can be potentially applied to other catalytic hydrogenolysis systems. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 756-767, 2010