A self-assembled monolayer of a ferrocenylalkanethiol, 11-ferrocenyl-1-undecanethiol (FcC(11)SH) formed on a gold electrode was investigated by a dynamic ellipsometry in 1 mol.dm(-3) perchloric acid and 0.5 mol.dm(-3) sulfuric acid solutions. The thickness of the molecular layer was evaluated from the ellipsometric measurement on a removal process of the layer. The removal was made by an anodic oxidation in the potential region of gold oxide film formation. The thickness of the FcC(11)SH layer was estimated to be 2.91 nm and the complex refractive index to be (n) over cap = 1.464-0.0741i at a light wavelength of 632.8 nm. The oxidation of the layer at a potential region lower than that of the oxide formation causes a thickness increase of the FcC(11)SH layer by 0.3 nm as a maximum and an increase of Light absorption by 10%. This change of the layer is explained by the oxidation from the ferrocenyl tail group to the ferricinium cation and a simultaneous uptake of anions from the electrolyte, resulting in an ion-pair formation between the anions and the ferricinium ions of the film.