The kinetics of C-I bond cleavage on the Ag(111) surface have been measured in a set of 10 alkyl and fluoroalkyl iodides. All the iodides adsorb molecularly on the Ag(111) surface at low temperatures and then dissociate during heating at temperatures below 200 K. X-ray photoemission spectroscopy was used to monitor the rate of C-I cleavage during heating. Estimation of the barrier to C-I cleavage (DeltaE(C-I)(double dagger)) indicates that it is sensitive to the nature of the alkyl or fluoroalkyl substituent groups bonded to the alpha -carbon atom. The barriers can be correlated to the field and polarizability substituent constants (sigma (F) and sigma (alpha)) of the alkyl groups and reveal that the reaction constants for C-I cleavage are quite low (rhoF = -17 +/- 1 kJ/mol and rho (alpha) = -11 +/- 2 kJ/mol). This suggests that the transition state for C-I cleavage is slightly anionic with respect to the adsorbed iodide (RC-I --> [RCdelta-. . .I](double dagger)) or, in other words, that the electron density on the carbon atom of the C-I bond is slightly greater in the transition state than in the reactant iodide. The magnitudes of the reaction constants, however, are relatively low. The implication is that the transition state to C-I cleavage is fairly homolytic in the sense that it occurs early in the reaction coordinate and is like the initial state iodide.