The build up and electrochemical characterization of interfacial composite nanostructures containing a cationic polyelectrolyte and negatively charged mercaptosuccinic acid stabilized gold nanoparticles (AuNPs) is reported. The nanostructures were formed at the interface between two immiscible electrolyte solutions in which the organic phase is an immobilized 2-nitrophenyl octyl ether/PVC gel. The growth of the multilayer was verified with UV-vis spectra, and approximately a linear increase in UV-vis absorbance with increasing number of layers was observed. The interfacial capacitance of the multilayers was measured as a function of the potential and a theoretical model was developed to explain the results. The excellent agreement between theoretical and experimental capacitance curves allows us to conclude that nanocomposites behave similarly to polyelectrolyte multilayers, with the outmost layer determining the alternating sign of the outer surface charge density. Cyclic voltammograms were used to evaluate the transfer rate constant across the multilayers of a model drug, metoprolol, and the standard probe tetraethylammonium cation. The apparent rate constants were slightly larger than in other studies in the literature and decrease with the increasing number of layers.