A complex electrochemical catalytic reaction was investigated at the interface between water and benzonitrile as a model for interfacial chemistry in microemulsions. Structures similar to those between oil-water microphases in microemulsions were created by using the interface between two immiscible electrolyte solutions (ITIES). While interfacial area in a microemulsion can be uncertain, the ITIES is well-defined and can be used to evaluate relevant heterogeneous inter-facial kinetics. The reaction between the Co(I) form of vitamin B-12 generated electrochemically in the water phase, and trans-1,2-dibromocyclohexane (DBCH) in benzonitrile was probed directly at the ITIES by scanning electrochemical microscopy (SECM). Apparent heterogeneous rate constants for the interfacial reaction were extracted from SECM current-distance curves, Influences of reactant concentration, potential drop across the ITIES, and adsorbed surfactants were investigated. Results suggest that the kinetics of reduction of DBCH by B-12 Co(I) are more complex at a liquid/liquid interface than that of the simple second-order rate-limiting process in a homogeneous organic solvent.