4-Nitrophenyl layers were grafted on gold and glassy carbon surfaces by electrochemical reductive adsorption of the corresponding diazonium salt. Electrochemical conversion efficiencies of 4-nitrophenyl moieties to 4-aminophenyl moieties on gold versus on glassy carbon in a protic medium were investigated using X-ray photoelectron spectroscopy (XPS). In total contrast to all previous comparative studies showing greater electrochemical reactivity of aryl diazonium salt-derived layers on gold than on glassy carbon, a much lower rate of conversion to 4-aminophenyl was observed on gold than on glassy carbon by both cyclic voltammetry (CV) and chronoamperometry (CA) methods. The lower electron transfer rate during conversion observed on gold versus glassy carbon was proposed to be due to a mechanism related to the molecular structure rearrangement of 4-nitrophenyl during the process on glassy carbon. However, whilst complete conversion of 4-nitrophenyl to 4-aminophenyl on gold by chronoamperometry was achieved, on glassy carbon complete reduction could not be achieved under the same conditions.