The composition of oxidic groups at a glassy-carbon surface has been studied using phase-sensitive ac-voltammetry. Two types of quinones have been identified, i.e. the 1, 2-naphtoquinone- and the 9, 10-phenanthrenequinone-like structures. The 1, 4-naphtoquinone- and 9, 10-anthraquinone-like structures are, most probably, also present at the glassy-carbon surface, although in lower surface concentrations. The o-quinones can be converted into the corresponding benzophenazines (by reaction with o-phenylenediamine), which are also electroactive. The differences in redox potentials between the quinones and phenazines make detection of intermediate reaction stages possible. The effect of oxygen and argon rf-plasma treatment upon the composition of the quinone-surface groups has also been studied. Pretreatment of the glassy-carbon surface by an oxygen rf-plasma is a very powerful and clean oxidation technique. Argon rf-plasma increases the 9, 10-phenanthrenequinone-like structures, while oxygen rf-plasma, with successive cooling in an argon atmosphere, diminishes the surface concentration of quinone structures and increases that of the acidic surface groups, as can be concluded from modification experiments. From phase-sensitive ac-voltammetry measurements, a lower limit of 103 s-1 was determined for both the quinone and the phenazine surface reaction rate constants (ks).