An intramolecular reaction catalyzed by Rh porphyrins was studied in the presence of interfacial electric fields. 1-Diazo-3,3-dimethyl-5-phenylhex-5-en-2-one (2) reacts with Rh porphyrins via a putative carbenoid intermediate to form cyclopropanation product 3,3-dimethyl-5-phenylbicyclo[3.1.0]hexan-2-one (3) and insertion product 3,3-dimethyl-2,3-dihydro-[1,1'-biphenyl]-4(1H)-one (4). To study this reaction in the presence of an interfacial electric field, Si electrodes coated with thin films of insulating dielectric layers were used as the opposing walls of a reaction vessel, and Rh porphyrin catalysts were localized to the dielectric-electrolyte interface. The charge density was varied at the interface by changing the voltage across the two electrodes. The product ratio was analyzed as a function of the applied voltage and the surface chemistry of the dielectric layer. In the absence of an applied voltage, the ratio of 3:4 was approximately 10:1. With a TiO2 surface, application of a voltage induced a Rh porphyrin-TiO2 interaction that resulted in an increase in the 3:4 ratio to a maximum in which 4 was nearly completely suppressed (>100:1). With an Al2O3 surface or an alkylphosphonate-coated surface, the voltage caused a decrease in the 3:4 ratio, with a maximum effect of lowering the ratio to 1:2. The voltage-induced decrease in the 3:4 ratio in the absence of TiO2 was consistent with a field-dipole effect that changed the difference in activation energies for the product-determining step to favor product 4. Effects were observed for porphyrin catalysts localized to the electrode-electrolyte interface either through covalent attachment or surface adsorption, enabling the selectivity to be controlled with unfunctionalized Rh porphyrins. The magnitude of the selectivity change was limited by the maximum interfacial charge density that could be attained before dielectric breakdown.