The electric double layer containing a negatively charged compact-layer surface was designed bymodifying the 3-mercaptopropionic acid self-assembled monolayer (3-MPA SAM) on the Au electrode. Thus the surface potential of the compact layer on the SAM can be adjusted by shifting the dissociation equilibrium of the carboxyl group (-COOH). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical reversibility of Fe(CN)(6)(3-/4-) at the above mentioned electrode, respectively. A key finding is that the negatively charged 3-MPA SAM surface can depress the electrochemical reversibility of Fe(CN)(6)(3-/4-), while the formation of ion-pairs between the dissociated carboxylate groups (-COO-) and the divalent cations on the monolayer can improve the electrochemical reversibility of Fe(CN)(6)(3-/4-) by increasing the surface potential of the compact layer. Further, the electrochemical reversibility of Fe(CN)(6)(3-/4-) can also be regulated by a mixed solution of different divalent cations. The logarithm of ion-pair formation constants (log K) with the -COO- were determined to be 2.78 +/- 0.03, 2.75 +/- 0.04, 2.69 +/- 0.04 for Ba2+, Ca2+, Mg2+ by electrochemical titration method, respectively. (C) 2016 The Electrochemical Society. All rights reserved.