The potential dependent structure change and mass transport at an 11-ferrocenyl-1-undecanethiol (FcC₁₁SH) self-assembled monolayer on a gold electrode with various coverages were investigated by simultaneous Fourier transform infrared reflection absorption spectroscopy (IRRAS) and electrochemical quartz crystal microbalance (EQCM) measurements in 0.1 M HClO₄ solution. As soon as the terminal ferrocene group was oxidized to a ferricenium cation (Fc⁺), a number of upward and downward IRRAS bands were observed and the surface mass was increased. The intensity of the IRRAS bands corresponded well with the mass change. This behavior is attributed to the redox reaction induced orientation change in the FcC₁₁SH monolayer and ion pair formation between the Fc⁺ cation and the perchlorate anion in solution. The FcC₁₁SH monolayer with a lower coverage was less stable and was decomposed at a less positive potential than that with a high coverage. Models are proposed to explain the coverage dependent behavior of the FcC₁₁SH monolayer induced by the redox reaction of the terminal ferrocene moiety.