Substituent effects have been used to probe the nature of the transition state to catalytic carbon-halogen bond breaking. Kinetics measurements have determined the activation energies (E-act) to C-Cl bond breaking on the Pd(111) surface and C-I bond breaking on the Pd(111) and Ag(111) surfaces. These barriers have been measured using alkyl halides with varying degrees of fluorine substitution. The activation energies have been correlated with the inductive or field substituent constants (sigma(F)) of the fluorinated alkyl groups in order to determine reaction constants (E-act= E-0 + rho sigma(F)) for the dehalogenation reactions. In all three cases it has been found that the barriers are insensitive to inductive substituent effects and the reaction constants are all relatively small: rho = -0.5 +/- 1.0 kcal/mol for C-Cl cleavage on Pd(111), rho = -0.3 +/- 0.8 kcal/mol for C-I cleavage on Pd(111), and rho = -2.9 +/- 0.4 kcal/mol for C-I cleavage on Ag(111). This implies that the transition state for dehalogenation is homolytic and occurs early in the reaction coordinate. The implications of this result are discussed for catalytic dehalogenation processes such as hydrodechlorination.