Cyclic voltammetry (CV) of [Ru(phen)₃]²⁺ (phen = 1,10-phenanthroline) is studied in CH₃CN under N₂ at a gold electrode whose surface is modified by thiocyanate. Five one-electron ligand reductions are observed on the negative side of normal hydrogen electrode (NHE), which is unprecedented. Using Randles-Sevcik equation, the electrochemical radius of the electrode has been estimated as 0.86 mm (cf. physical radius: 1 mm). When the Au-SCN electrode is dipped in an CH₃CN solution of [Ru(phen)₃]²⁺ for 2 h, the cation gets adsorbed on the electrode surface. The resulting Au-SCN-Ru(phen)₃²⁺ electrode shows two reductions in plain CH₃CN under N₂. The number of electrons in each step is found to be 2 when Laviron’s equation is applied with a transfer coefficient α of 0.5. Ferrocene at the Au-SCN-Ru(phen)₃²⁺ electrode in CH₃CN shows a one-electron oxidation at 0.681 V vs NHE. The radius of the electrode comes out as 1.02 mm. The heterogeneous rate constant k_s,h for the oxidation of ferrocene is found to be 2.77 × 10⁻² cm s⁻¹ and that at an unmodified Au electrode in CH₃CN 1.03 × 10⁻² cm s⁻¹. When CV is performed in CH₃CN in air, the Au-SCN-Ru(phen)₃²⁺ electrode shows electro-catalytic reductions of dioxygen (O₂ + e → O₂⁻ and O₂⁻ + e → O₂²⁻). The k_s,h of the first step is found to be 2.99 × 10⁻³ cm s⁻¹ by Nicholson’s method (with α = 0.306). This k_s,h at an unmodified Au electrode is 1.21 × 10⁻³ cm s⁻¹. In course of the studies, it has been necessary to determine the X-ray crystal structures of [Ru(phen)₃](NCS)₂ and [Ru(phen)₃]₂[Fe(CN)₆]Cl·18.5H₂O.