Reverse electron transfer, in which an electron is transferred uphill from a redox couple with a higher standard reduction potential in one phase to another redox couple having a lower standard reduction potential in a second immiscible phase, is demonstrated using the system TCNQ (in 1,2-dichloroethane)/ferrocyanide (in water). The driving force for the reverse electron transfer is the presence of appropriate potential-determining ions that govern the interfacial potential difference, which in turn determines the position of equilibrium in the two-phase redox reaction. The occurrence of reverse electron transfer was monitored by scanning electrochemical microscopy.