To treat the problem of oxalate oxidation in an aqueous phase by a Ru(III) species (mediator) generated in an immiscible nonaqueous phase (benzonitrile) at a scanning electrochemical microscope (SECM tip), the theory of the feedback mode of the SECM is extended to include both finite heterogeneous electron transfer (ET) kinetics at the substrate and homogeneous decomposition of the mediator. As for the classical electrochemical (EC scheme, a zone diagram is constructed showing pure ET heterogeneous kinetic control, pure homogeneous kinetic (C) control (insulating behavior), and mixed EC kinetic control. The model allows interpretation of the anomalous approach curves obtained for oxalate oxidation by a ruthenium(III) coordination complex at the benzonitrile (BN)-water interface and allows calculation of the rate constant for the ET at the liquid-liquid interface. Attempts are made to relate these rates to the homogeneous rate constant in terms of Marcus theory. The second ET (oxidation of CO2. (-)) at the liquid-liquid interface generates an emitting excited state of the ruthenium species. The electrogenerated chemiluminescence process is related to the current crossing the interface.