Scanning electrochemical microscopy (SECM) was used to investigate the kinetics of heterogeneous electron transfer (ET) as a function of driving force at the interface between two immiscible electrolyte solutions. At high driving force, experimental rate constants decreased with increasing overpotential, deviating from predictions based on Butler-Volmer kinetics. This decrease in ET rate with increasing driving force is consistent with Marcus theory inverted region behavior. Ar low driving force, the potential dependence of the forward and reverse ET rate constants followed Butler-Volmer theory. SECM is also demonstrated to be a useful means of studying the effect of high ionic strength on the kinetics of heterogeneous ET.