Substituent effects have been used as a means of probing the nature of the transition state for C-I bond cleavage on the Pd(111) surface. The barriers to C-I cleavage (DeltaE(C-I)(double dagger)) have been measured in a set of 10 different alkyl and fluoroalkyl iodides (CH3I, CF3I, CH3CH2I, CF3CH2I, CF2HCF2I, CH3CH2CH2I, CF3CH2CH2I, CF3CF2CH2I, (CH3)(2)CHI, and (CH3)(3)CI) on Pd(111). These measurements were performed by adsorbing the iodides on the Pd(111) surface at low temperature (90 K) and then heating to 250 K to induce dissociation (R-I-(ad) --> R-(ad) + I-(ad)). X-ray photoemission of the I 3d(5/2) level was used to monitor the extent of reaction C-1 the different alkyl and flu-during heating. To influence DeltaE(C-I)(double dagger) oroalkyl groups were chosen to give a wide range of field effect (sigma (F)) substituent constants. By correlating DeltaE(C-I)(double dagger) with the field effect through a linear free energy relationship (Delta DeltaE(C-I)(double dagger) = rho (F) . sigma (F)) it has been possible to compare the activation of C-I bonds on the Pd(111) surface with other dehalogenation reactions (C-Cl cleavage on Pd(111) and C-I cleavage on Ag(111)). In all cases the reaction constants (rho (F)) are very small. For C-I cleavage on the Pd(111) surface rho (F) = 0. These results indicate that the transition state to C-I cleavage is homolytic in the sense that it occurs early in the reaction coordinate and the reaction center in the transition state [C (...) I](double dagger) is not much different from the initial state reactant. This result appears to be generally true of metal catalyzed dehalogenation reactions.