A new mu-oxo-bis(mu-acetato)diruthenium(III) complex bearing two pyridyl disulfide ligands {[Ru-2(mu-O)(mu-OAc)(2)(bpy)(2)(L-py center dot SS)(2)](PF6)(2) (OAc = CH3CO2-, bpy = 2,2'-bipyridine, L-py center dot SS = (C5H4N)CH2NHC(O)(CH2)(4)CH(CH2)(2)SS) (1)} has been synthesized to prepare self-assembled monolayers (SAMs) on the Au(111) electrode surface. The SAMs have been characterized by contact-angle measurements, reflection absorption surface infrared spectroscopy, cyclic voltammetry, and reductive desorption experiments. The SAMs exhibited proton-coupled electron transfer (PCET) reactions when the electrochemistry was studied in aqueous electrolyte solution (0.1 M NaClO4 with Britton Robinson buffer to adjust the solution pH). The potential-pH plot (Pourbaix diagram) in the pH range from 1 to 12 has established that the dinuc.lear ruthenium moiety was involved in the interfacial PCET processes with four distinct redox states: (RuRuIII)-Ru-III(mu-O), (RuRuIII)-Ru-II(mu-OH), (RuRuII)-Ru-II(mu-OH), and (RuRuII)-Ru-II(mu-OH2). We also demonstrated that the interfacial redox processes were modulated by the addition of Lewis acids such as BF3 or Al3+ to the electrolyte media, in which the externally added Lewis acids interacted with mu-O of the dinuclear moiety within the SAMs.