The electrochemical properties of a monolayer graphene grown on a Au(111) electrode were studied using cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). CV and EC-STM measurements in 0.1 M H2SO4 aqueous solution revealed that graphene grown on the reconstructed (22 x root 3) Au(111) structure effectively inhibited potential-induced structural transitions between reconstructed (22 x root 3) and unreconstructed (1 x 1), and the adsorption/desorption of SO42- ions, which are intrinsic behavior of the bare Au(111) surface. The underlying reconstructed structure was significantly stabilized by covering with monolayer graphene over a wide potential range between -0.2 V and +1.35 V vs Ag/AgCl (saturated KCl), which is much wider than that for bare Au(111) (-0.2 to + 0.35 V vs Ag/AgCl (saturated KO)). Such high stability has not been reported to date; therefore, these results are considered to be important for understanding the fundamentals of surface reconstruction and also serve to open a new branch of electrochemistry related to graphene/metal electrolyte interfaces.