Electrocatalytic activity of a new catalyst toward the oxidation reaction of hydroquinone as a model compound is described. The catalyst was formed by immobilizing metal cations on the topside of a gold-5-amino-2-mercaptobenzimidazole, self-assembled monolayer (Au-5A2MBI-M n+ SAM, M n+: Cu2+, Ag+) electrode. Preparation steps and the electrocatalytic activity of the catalyst were studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The EIS data were approximated by appropriate electronic equivalent circuit models from which kinetic parameters, such as charge transfer resistance, double layer capacitance, and apparent rate constant (k app), were estimated. Excellent activity was observed for Au-5A2MBI-Ag+ SAM with the following order: Au-5A2MBI-Ag+ >Au-5A2MBI-Cu2+ >Au-5A2MBI, after testing many modified electrodes. The increased activity originates from a modification of the Au-5A2MBI structure by mediating the effect of Ag+. This behavior was understood from significant increases in the k app without significant changes in the double layer capacitance.