The decay kinetics of the photovoltage formed on pulsed illumination of a chlorophyll a- (chl a-) containing lecithin-bilayer adjacent to a ferricyanide solution on one side show characteristics of a system with distributed rate constants, i.e., the decay approaches linearity in log of time. The kinetics can be explained by a distribution of the chl cation over a few angstroms depth in the interfacial region of the bilayer and a rate constant exponentially dependent on distance as expected from tunneling theory. Addition of the donor ferrocyanide both increases the average rate and sharpens the distribution. There is a competitive inhibition by ferricyanide of the reaction of pigment cation with ferrocyanide. Removal of oxygen increases the rate of decay when an acceptor, methyl viologen or anthraquinone-2-sulfonate, forms oxygen-sensitive radicals. The cation charge does not cross the bilayer on a time scale of less than 0.01 s. These data define a reaction localized precisely in the finite interfacial region of the lipid bilayer-water interface.