The standard electron-transfer rate constants (k(0)) are measured for redox processes of Fe versus Zn porphyrins in monolayers on An(111); the former undergoes a metal-centered redox process (conversion between Fe-III and Fe-II oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzy] thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu4NCl) conditions, the Zn-II center has a coordinated Cl- ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed-a low-potential form (E-l(0) approximate to -0.6 V) wherein the metal center has a coordinated Cl- ion when it is in either the Fe-II or Fe-III state and a high-potential form (E-h(0) approximate to +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe-III to Fe-II states. The k(0) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k(0) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k(0) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl- exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.