The effect of a 4-carboxyphenyl or a 4-nitrophenyl thin film at the surface of a glassy carbon electrode on their electrochemical responses in the presence of various electroactive probes has been investigated. The grafting of a substituted phenyl group to a glassy carbon electrode was achieved by electrochemical reduction of the corresponding substituted phenyldiazonium derivative in acetonitrile. The blocking properties of the film depend primarily on electrostatic and electrolyte/solvent effects. Permselectivity for the 4-carboxyphenyl film can be achieved by controlling the dissociation of the carboxy group. The substituted phenyl layer is much more compact and less permeable in contact with a nonaqueous solvent than with an aqueous solvent presumably because the layer is poorly solvated. Electrochemical impedance measurements indicate that the kinetics of electron transfer are slowed down when the time used to modify the glassy carbon electrode is increased. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements for 4-nitrophenyl- or 4-carboxyphenyl-modified glassy carbon electrode have indicated close to monolayer coverage for the substituted phenyl groups. The presence of a peak at 400 eV on the nitrogen Is core level spectra was tentatively attributed to the reaction of phenol groups present at the glassy carbon electrode surface with the diazonium salt since this peak is not observed for the unmodified glassy carbon electrode.