Synthesis and reduction of diazonium cations in acetonitrile - without isolation of the solid - was carried out to modify glassy carbon (GC) electrodes with p-nitrophenyl or p-carboxyphenyl groups. Reduction of diazonium ions was investigated under a variety of conditions to further understand the process. Cyclic voltammetry at GC consistently yields two distinct peaks for the reduction of nitrobenzenediazonium ions; the first peak is particularly dependent upon the GC surface treatment and history. Subsequent reduction of the surface nitrophenyl groups was used to assess the extent of modification, and results were compared for different electrografting conditions. Electrolysis of diazonium ions, using a more positive reduction potential than typical, results in modified surfaces, and the electrolysis charge can be varied to affect the surface coverage of modifying groups, though with %RSD > 25%. Electrochemistry of common redox systems (dopamine, ruthenium hexammine, ferrocyanide) was studied as a function of modification parameters. Under mild electrografting conditions, it appears that inhomogeneously covered surfaces are produced, with blocking properties similar to spontaneously grafted GC. Additionally, electron transfer for Fe(2+/3+) is suppressed at carboxyphenyl-modified GC rather than accelerated as it is at oxidized carbon electrodes containing carbonyl groups. (C) 2009 Elsevier B.V. All rights reserved.