Soft or "liquid-liquid" interfaces were functionalized by roughly half a monolayer of mirror-like nanofilms of gold nanoparticles using a precise interfacial microinjection method. The surface coverage of the nanofilm was characterized by ion transfer voltammetry. These gold nanoparticle films represent an ideal model system for studying both the thermodynamic and kinetic aspects of interfacial redox catalysis. The electric polarization of these soft interfaces is easily controllable, and thus the Fermi level of the electrons in the interfacial gold nanoparticle film can be easily manipulated. Here, we study interfacial redox catalysis between two redox couples located in adjacent immiscible phases and highlight the catalytic properties of a gold nanoparticle film toward heterogeneous electron transfer reactions.