Electroactive ferrocenylundecanethiol (FcC(11)H(22)SH) islands embedded in an n-decanethiol (C10H21SH) self-assembled monolayer (SAM) matrix on Au(111) were studied under potential control in 0.1 M HClO4 aqueous solution using newly developed electrochemical frequency-modulation atomic force microscopy (EC-FM-AFM). The apparent height of the Fc islands from the surface of the matrix SAM increased by about 0.44 nm accompanied by the oxidation of the terminal Fc groups. This potential-dependent reversible change can be explained by formation of an ionic double layer where ClO4- ions are strongly bound on the Fc(+) groups. Simultaneous measurements of energy dissipation, which corresponds to the energy to keep the cantilever's vibrational amplitude constant, revealed distinct change in the magnitude by the oxidation state of the Fc groups. These results indicate that localized charge at an electrode/electrolyte solution interface can be identified, and microscopic in on the electric double layer at the interface is available by using EC-FM-AFM.