Electrochemical and spectroscopic investigations were performed in order to clarify the mechanism of 4-nitrobenzene diazonium reduction on glassy carbon in protic medium. The number and nature of the electron transfer processes were found to be strongly correlated to the electrode surface state. On polished electrode two different reduction peaks were observed. Selective electrolyses realized at the corresponding potential definitely proved that the grafting process actually occurs at a potential distinct from NO2 electroreduction, this latter inducing the presence of the quasi-reversible NO/NHOH redox couple at the electrode surface. These results were confirmed by XPS analyses. Furthermore, voltammetric experiments using Fe(CN)63- showed that the electrochemical properties of the modified electrode strongly depend on the potential applied for grafting, which modulates the nitro group oxidation state. All the results suggested that the electrode functionalization was more efficient when grafting and NO2 reduction were performed separately.