The derivatization of a glassy carbon electrode surface was achieved by electrochemical reduction of two phenyl diazonium salts in aqueous and acetonitrile solutions. The effect of diazonium thin films at the surface of a glassy carbon electrode on their electrochemical responses in the presence of Fe(CN)(6)(3-/4-) probe have been investigated. The substituted phenyl layers are much more compact and less permeable in a nonaqueous solvent than with an aqueous solvent. Electrochemical impedance measurements indicate that the kinetics of electron transfer is showed down when the time and the concentration used to modify the glassy carbon electrode are increased. Also, electron transfer processes of Fe(CN)(6)(3-/4-) in solutions of various pHs at the modified electrode are studied. Changing the solution pH would result in the terminal group's charge state, based on which the surface pK(a) values are estimated. (C) 2008 Elsevier B.V. All rights reserved.