Redox behaviors of binary self-assembled monolayers (SAMs) consisting of ferrocenyl- and methyl-terminated alkanethiols on gold were investigated in three different ionic liquids (ILs) and were compared with the behaviors in an aqueous HClO(4) solution. A set of cyclic voltammograms showed that, when the surface density of ferrocenyl-groups was relatively high, the shape of redox waves (e.g. peak area and peak broadening towards positive potential) for the ferrocenyl-groups was strongly affected by the anion size of the ILs, suggesting that the compensation of surface positive charge emerged when oxidized to ferrocenium state is sterically-hindered in the case of large anions like bis{(trifluoromethyl)sulfonyll]amide (TFSA(-)). This is supported by the fact that the anion-size effect disappeared if the density of ferrocenyl-groups was diluted. A potential-step chronocoulometry was concomitantly employed to estimate the net faradaic charge of the redox. The redox behaviors in mixed ILs containing two different sized anions also gave clear evidence that smaller anions, e.g. BF(4)(-) ions, have a greater tendency to make ion pairs with ferrocenium moieties. (C) 2010 Elsevier B.V. All rights reserved.