The electrochemical modification of the gold electrode surface with various phenyl groups were performed by the diazonium reduction method in aqueous and non-aqueous solutions. The reduction of these salts in two medium permits the attachment of substituted groups to the gold surface. The presence of phenyl groups at the electrode surface was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). These techniques were used to study the influence of the parameters such as reduction time, concentration of diazonium salts, and applied media on the electrodeposition at constant potential. The barrier properties of phenyl modified gold surfaces were characterized using Fe(CN)(6)(3-/4-) as a redox probe. The results are indicative of the presence of substituted phenyl groups on Au surface. The slowing down of the electron-transfer kinetics was more evident by increasing the time and concentration as indicated by the increase of both Delta E(p) and R(CT). The results show that grafted layers on the gold electrode surface in aqueous solution are less compact than that grafted in acetonitrile solution. (C) 2010 Elsevier B.V. All rights reserved.