The electron transfer (ET) rate at the interface between two immiscible electrolyte solutions was probed as a function of the driving force and distance between redox centers by scanning electrochemical microscopy. The adsorption of phospholipids at the interface resulted in a decrease in the rate of interfacial ET between aqueous redox species and the oxidized form of zinc porphyrin in benzene. The fraction of the interfacial area covered with lipid (theta) was evaluated from the measured heterogeneous rate constants (k(f)). The dependence of theta vs lipid concentration in benzene fit a Langmuir isotherm. For complete monolayers of phospholipids, k(f) was a function of the number of methylene groups in a hydrocarbon chain. The driving force dependencies of interfacial ET rates (Tafel plots) were measured for several aqueous redox couples. They were linear; with a transfer coefficient of alpha similar or equal to 0.5 when the driving force for ET (Delta G degrees) was not too high, in agreement with Marcus theory, and leveled off to the diffusion-controlled rate at larger overpotentials. For even higher Delta G degrees and for the first time for heterogeneous ET at a polarizable interface, inverted region behavior was observed.